Solve {l}{6x-7y=24-3x}{4x+7y=x+6y+18}

$$\left. \begin{array} { l } { 6 x - 7 y = 24 - 3 x } \\ { 4 x + 7 y = x + 6 y + 18 } \end{array} \right.$$

Solve for x, y

$$x=5$$

$$y=3$$

Steps Using Substitution

Consider the first equation. Add 3x to both sides.

$$6x-7y+3x=24$$

Combine 6x and 3x to get 9x.

$$9x-7y=24$$

Consider the second equation. Subtract x from both sides.

$$4x+7y-x=6y+18$$

Combine 4x and -x to get 3x.

$$3x+7y=6y+18$$

Subtract 6y from both sides.

$$3x+7y-6y=18$$

Combine 7y and -6y to get y.

$$3x+y=18$$

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.

$$9x-7y=24,3x+y=18$$

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

$$9x-7y=24$$

Add 7y to both sides of the equation.

$$9x=7y+24$$

Divide both sides by 9.

$$x=\frac{1}{9}\left(7y+24\right)$$

Multiply \frac{1}{9} times 7y+24.

$$x=\frac{7}{9}y+\frac{8}{3}$$

Substitute \frac{7y}{9}+\frac{8}{3} for x in the other equation, 3x+y=18.

$$3\left(\frac{7}{9}y+\frac{8}{3}\right)+y=18$$

Multiply 3 times \frac{7y}{9}+\frac{8}{3}.

$$\frac{7}{3}y+8+y=18$$

Add \frac{7y}{3} to y.

$$\frac{10}{3}y+8=18$$

Subtract 8 from both sides of the equation.

$$\frac{10}{3}y=10$$

Divide both sides of the equation by \frac{10}{3}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$y=3$$

Substitute 3 for y in x=\frac{7}{9}y+\frac{8}{3}. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=\frac{7}{9}\times 3+\frac{8}{3}$$

Multiply \frac{7}{9} times 3.

$$x=\frac{7+8}{3}$$

Add \frac{8}{3} to \frac{7}{3} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x=5$$

The system is now solved.

$$x=5,y=3$$

Steps Using Matrices

Consider the first equation. Add 3x to both sides.

$$6x-7y+3x=24$$

Combine 6x and 3x to get 9x.

$$9x-7y=24$$

Consider the second equation. Subtract x from both sides.

$$4x+7y-x=6y+18$$

Combine 4x and -x to get 3x.

$$3x+7y=6y+18$$

Subtract 6y from both sides.

$$3x+7y-6y=18$$

Combine 7y and -6y to get y.

$$3x+y=18$$

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

$$9x-7y=24,3x+y=18$$

Write the equations in matrix form.

$$\left(\begin{matrix}9&-7\\3&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}24\\18\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}9&-7\\3&1\end{matrix}\right).

$$inverse(\left(\begin{matrix}9&-7\\3&1\end{matrix}\right))\left(\begin{matrix}9&-7\\3&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}9&-7\\3&1\end{matrix}\right))\left(\begin{matrix}24\\18\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}9&-7\\3&1\end{matrix}\right))\left(\begin{matrix}24\\18\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}9&-7\\3&1\end{matrix}\right))\left(\begin{matrix}24\\18\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{9-\left(-7\times 3\right)}&-\frac{-7}{9-\left(-7\times 3\right)}\\-\frac{3}{9-\left(-7\times 3\right)}&\frac{9}{9-\left(-7\times 3\right)}\end{matrix}\right)\left(\begin{matrix}24\\18\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{30}&\frac{7}{30}\\-\frac{1}{10}&\frac{3}{10}\end{matrix}\right)\left(\begin{matrix}24\\18\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{1}{30}\times 24+\frac{7}{30}\times 18\\-\frac{1}{10}\times 24+\frac{3}{10}\times 18\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}5\\3\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=5,y=3$$

Steps Using Elimination

Consider the first equation. Add 3x to both sides.

$$6x-7y+3x=24$$

Combine 6x and 3x to get 9x.

$$9x-7y=24$$

Consider the second equation. Subtract x from both sides.

$$4x+7y-x=6y+18$$

Combine 4x and -x to get 3x.

$$3x+7y=6y+18$$

Subtract 6y from both sides.

$$3x+7y-6y=18$$

Combine 7y and -6y to get y.

$$3x+y=18$$

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.

$$9x-7y=24,3x+y=18$$

To make 9x and 3x equal, multiply all terms on each side of the first equation by 3 and all terms on each side of the second by 9.

$$3\times 9x+3\left(-7\right)y=3\times 24,9\times 3x+9y=9\times 18$$

Simplify.

$$27x-21y=72,27x+9y=162$$

Subtract 27x+9y=162 from 27x-21y=72 by subtracting like terms on each side of the equal sign.

$$27x-27x-21y-9y=72-162$$

Add 27x to -27x. Terms 27x and -27x cancel out, leaving an equation with only one variable that can be solved.

$$-21y-9y=72-162$$

Add -21y to -9y.

$$-30y=72-162$$

Add 72 to -162.

$$-30y=-90$$

Divide both sides by -30.

$$y=3$$

Substitute 3 for y in 3x+y=18. Because the resulting equation contains only one variable, you can solve for x directly.

$$3x+3=18$$

Subtract 3 from both sides of the equation.

$$3x=15$$

Divide both sides by 3.

$$x=5$$

The system is now solved.

$$x=5,y=3$$