Solve {l}{3y=-9x+9}{-2x+2y=6}

$$\left\{ \begin{array} { l } { 3 y = - 9 x + 9 } \\ { - 2 x + 2 y = 6 } \end{array} \right.$$

Solve for y, x

$$x=0$$

$$y=3$$

Steps Using Substitution

Consider the first equation. Add 9x to both sides.

$$3y+9x=9$$

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.

$$3y+9x=9,2y-2x=6$$

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

$$3y+9x=9$$

Subtract 9x from both sides of the equation.

$$3y=-9x+9$$

Divide both sides by 3.

$$y=\frac{1}{3}\left(-9x+9\right)$$

Multiply \frac{1}{3} times -9x+9.

$$y=-3x+3$$

Substitute -3x+3 for y in the other equation, 2y-2x=6.

$$2\left(-3x+3\right)-2x=6$$

Multiply 2 times -3x+3.

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

Add -6x to -2x.

$$-8x+6=6$$

Subtract 6 from both sides of the equation.

$$-8x=0$$

Divide both sides by -8.

$$x=0$$

Substitute 0 for x in y=-3x+3. Because the resulting equation contains only one variable, you can solve for y directly.

$$y=3$$

The system is now solved.

$$y=3,x=0$$

Steps Using Matrices

Consider the first equation. Add 9x to both sides.

$$3y+9x=9$$

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

$$3y+9x=9,2y-2x=6$$

Write the equations in matrix form.

$$\left(\begin{matrix}3&9\\2&-2\end{matrix}\right)\left(\begin{matrix}y\\x\end{matrix}\right)=\left(\begin{matrix}9\\6\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}3&9\\2&-2\end{matrix}\right).

$$inverse(\left(\begin{matrix}3&9\\2&-2\end{matrix}\right))\left(\begin{matrix}3&9\\2&-2\end{matrix}\right)\left(\begin{matrix}y\\x\end{matrix}\right)=inverse(\left(\begin{matrix}3&9\\2&-2\end{matrix}\right))\left(\begin{matrix}9\\6\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}y\\x\end{matrix}\right)=inverse(\left(\begin{matrix}3&9\\2&-2\end{matrix}\right))\left(\begin{matrix}9\\6\end{matrix}\right)$$

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

$$\left(\begin{matrix}y\\x\end{matrix}\right)=inverse(\left(\begin{matrix}3&9\\2&-2\end{matrix}\right))\left(\begin{matrix}9\\6\end{matrix}\right)$$

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.

$$\left(\begin{matrix}y\\x\end{matrix}\right)=\left(\begin{matrix}-\frac{2}{3\left(-2\right)-9\times 2}&-\frac{9}{3\left(-2\right)-9\times 2}\\-\frac{2}{3\left(-2\right)-9\times 2}&\frac{3}{3\left(-2\right)-9\times 2}\end{matrix}\right)\left(\begin{matrix}9\\6\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}y\\x\end{matrix}\right)=\left(\begin{matrix}\frac{1}{12}&\frac{3}{8}\\\frac{1}{12}&-\frac{1}{8}\end{matrix}\right)\left(\begin{matrix}9\\6\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}y\\x\end{matrix}\right)=\left(\begin{matrix}\frac{1}{12}\times 9+\frac{3}{8}\times 6\\\frac{1}{12}\times 9-\frac{1}{8}\times 6\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}y\\x\end{matrix}\right)=\left(\begin{matrix}3\\0\end{matrix}\right)$$

Extract the matrix elements y and x.

$$y=3,x=0$$

Steps Using Elimination

Consider the first equation. Add 9x to both sides.

$$3y+9x=9$$

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.

$$3y+9x=9,2y-2x=6$$

To make 3y and 2y equal, multiply all terms on each side of the first equation by 2 and all terms on each side of the second by 3.

$$2\times 3y+2\times 9x=2\times 9,3\times 2y+3\left(-2\right)x=3\times 6$$

Simplify.

$$6y+18x=18,6y-6x=18$$

Subtract 6y-6x=18 from 6y+18x=18 by subtracting like terms on each side of the equal sign.

$$6y-6y+18x+6x=18-18$$

Add 6y to -6y. Terms 6y and -6y cancel out, leaving an equation with only one variable that can be solved.

$$18x+6x=18-18$$

Add 18x to 6x.

$$24x=18-18$$

Add 18 to -18.

$$24x=0$$

Divide both sides by 24.

$$x=0$$

Substitute 0 for x in 2y-2x=6. Because the resulting equation contains only one variable, you can solve for y directly.

$$2y=6$$

Divide both sides by 2.

$$y=3$$

The system is now solved.

$$y=3,x=0$$