Solve {l}{2(y+6)=x-1}{x+6=3(1-2y)}

$$\left. \begin{array} { l } { 2 ( y + 6 ) = x - 1 } \\ { x + 6 = 3 ( 1 - 2 y ) } \end{array} \right.$$

Solve for y, x

$$x=9$$

$$y=-2$$

Steps Using Substitution

Consider the first equation. Use the distributive property to multiply 2 by y+6.

$$2y+12=x-1$$

Subtract x from both sides.

$$2y+12-x=-1$$

Subtract 12 from both sides.

$$2y-x=-1-12$$

Subtract 12 from -1 to get -13.

$$2y-x=-13$$

Consider the second equation. Use the distributive property to multiply 3 by 1-2y.

$$x+6=3-6y$$

Add 6y to both sides.

$$x+6+6y=3$$

Subtract 6 from both sides.

$$x+6y=3-6$$

Subtract 6 from 3 to get -3.

$$x+6y=-3$$

To solve a pair of equations using substitution, first solve one of the equations for one of the variables. Then substitute the result for that variable in the other equation.

$$2y-x=-13,6y+x=-3$$

Choose one of the equations and solve it for y by isolating y on the left hand side of the equal sign.

$$2y-x=-13$$

Add x to both sides of the equation.

$$2y=x-13$$

Divide both sides by 2.

$$y=\frac{1}{2}\left(x-13\right)$$

Multiply \frac{1}{2} times x-13.

$$y=\frac{1}{2}x-\frac{13}{2}$$

Substitute \frac{-13+x}{2} for y in the other equation, 6y+x=-3.

$$6\left(\frac{1}{2}x-\frac{13}{2}\right)+x=-3$$

Multiply 6 times \frac{-13+x}{2}.

$$3x-39+x=-3$$

Add 3x to x.

$$4x-39=-3$$

Add 39 to both sides of the equation.

$$4x=36$$

Divide both sides by 4.

$$x=9$$

Substitute 9 for x in y=\frac{1}{2}x-\frac{13}{2}. Because the resulting equation contains only one variable, you can solve for y directly.

$$y=\frac{1}{2}\times 9-\frac{13}{2}$$

Multiply \frac{1}{2} times 9.

$$y=\frac{9-13}{2}$$

Add -\frac{13}{2} to \frac{9}{2} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$y=-2$$

The system is now solved.

$$y=-2,x=9$$

Steps Using Matrices

Consider the first equation. Use the distributive property to multiply 2 by y+6.

$$2y+12=x-1$$

Subtract x from both sides.

$$2y+12-x=-1$$

Subtract 12 from both sides.

$$2y-x=-1-12$$

Subtract 12 from -1 to get -13.

$$2y-x=-13$$

Consider the second equation. Use the distributive property to multiply 3 by 1-2y.

$$x+6=3-6y$$

Add 6y to both sides.

$$x+6+6y=3$$

Subtract 6 from both sides.

$$x+6y=3-6$$

Subtract 6 from 3 to get -3.

$$x+6y=-3$$

Put the equations in standard form and then use matrices to solve the system of equations.

$$2y-x=-13,6y+x=-3$$

Write the equations in matrix form.

$$\left(\begin{matrix}2&-1\\6&1\end{matrix}\right)\left(\begin{matrix}y\\x\end{matrix}\right)=\left(\begin{matrix}-13\\-3\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}2&-1\\6&1\end{matrix}\right).

$$inverse(\left(\begin{matrix}2&-1\\6&1\end{matrix}\right))\left(\begin{matrix}2&-1\\6&1\end{matrix}\right)\left(\begin{matrix}y\\x\end{matrix}\right)=inverse(\left(\begin{matrix}2&-1\\6&1\end{matrix}\right))\left(\begin{matrix}-13\\-3\end{matrix}\right)$$

The product of a matrix and its inverse is the identity matrix.

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}y\\x\end{matrix}\right)=inverse(\left(\begin{matrix}2&-1\\6&1\end{matrix}\right))\left(\begin{matrix}-13\\-3\end{matrix}\right)$$

Multiply the matrices on the left hand side of the equal sign.

$$\left(\begin{matrix}y\\x\end{matrix}\right)=inverse(\left(\begin{matrix}2&-1\\6&1\end{matrix}\right))\left(\begin{matrix}-13\\-3\end{matrix}\right)$$

For the 2\times 2 matrix \left(\begin{matrix}a&b\\c&d\end{matrix}\right), the inverse matrix is \left(\begin{matrix}\frac{d}{ad-bc}&\frac{-b}{ad-bc}\\\frac{-c}{ad-bc}&\frac{a}{ad-bc}\end{matrix}\right), so the matrix equation can be rewritten as a matrix multiplication problem.

$$\left(\begin{matrix}y\\x\end{matrix}\right)=\left(\begin{matrix}\frac{1}{2-\left(-6\right)}&-\frac{-1}{2-\left(-6\right)}\\-\frac{6}{2-\left(-6\right)}&\frac{2}{2-\left(-6\right)}\end{matrix}\right)\left(\begin{matrix}-13\\-3\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}y\\x\end{matrix}\right)=\left(\begin{matrix}\frac{1}{8}&\frac{1}{8}\\-\frac{3}{4}&\frac{1}{4}\end{matrix}\right)\left(\begin{matrix}-13\\-3\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}y\\x\end{matrix}\right)=\left(\begin{matrix}\frac{1}{8}\left(-13\right)+\frac{1}{8}\left(-3\right)\\-\frac{3}{4}\left(-13\right)+\frac{1}{4}\left(-3\right)\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}y\\x\end{matrix}\right)=\left(\begin{matrix}-2\\9\end{matrix}\right)$$

Extract the matrix elements y and x.

$$y=-2,x=9$$

Steps Using Elimination

Consider the first equation. Use the distributive property to multiply 2 by y+6.

$$2y+12=x-1$$

Subtract x from both sides.

$$2y+12-x=-1$$

Subtract 12 from both sides.

$$2y-x=-1-12$$

Subtract 12 from -1 to get -13.

$$2y-x=-13$$

Consider the second equation. Use the distributive property to multiply 3 by 1-2y.

$$x+6=3-6y$$

Add 6y to both sides.

$$x+6+6y=3$$

Subtract 6 from both sides.

$$x+6y=3-6$$

Subtract 6 from 3 to get -3.

$$x+6y=-3$$

In order to solve by elimination, coefficients of one of the variables must be the same in both equations so that the variable will cancel out when one equation is subtracted from the other.

$$2y-x=-13,6y+x=-3$$

To make 2y and 6y equal, multiply all terms on each side of the first equation by 6 and all terms on each side of the second by 2.

$$6\times 2y+6\left(-1\right)x=6\left(-13\right),2\times 6y+2x=2\left(-3\right)$$

Simplify.

$$12y-6x=-78,12y+2x=-6$$

Subtract 12y+2x=-6 from 12y-6x=-78 by subtracting like terms on each side of the equal sign.

$$12y-12y-6x-2x=-78+6$$

Add 12y to -12y. Terms 12y and -12y cancel out, leaving an equation with only one variable that can be solved.

$$-6x-2x=-78+6$$

Add -6x to -2x.

$$-8x=-78+6$$

Add -78 to 6.

$$-8x=-72$$

Divide both sides by -8.

$$x=9$$

Substitute 9 for x in 6y+x=-3. Because the resulting equation contains only one variable, you can solve for y directly.

$$6y+9=-3$$

Subtract 9 from both sides of the equation.

$$6y=-12$$

Divide both sides by 6.

$$y=-2$$

The system is now solved.

$$y=-2,x=9$$