Solve {l}{-2x-3y=7}{-13x+20y=322}

$$\left. \begin{array}{l}{ - 2 x - 3 y = 7 }\\{ - 13 x + 20 y = 322 }\end{array} \right.$$

Solve for x, y

$$x=-14$$

$$y=7$$

Steps Using Substitution

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.

$$-2x-3y=7,-13x+20y=322$$

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

$$-2x-3y=7$$

Add 3y to both sides of the equation.

$$-2x=3y+7$$

Divide both sides by -2.

$$x=-\frac{1}{2}\left(3y+7\right)$$

Multiply -\frac{1}{2} times 3y+7.

$$x=-\frac{3}{2}y-\frac{7}{2}$$

Substitute \frac{-3y-7}{2} for x in the other equation, -13x+20y=322.

$$-13\left(-\frac{3}{2}y-\frac{7}{2}\right)+20y=322$$

Multiply -13 times \frac{-3y-7}{2}.

$$\frac{39}{2}y+\frac{91}{2}+20y=322$$

Add \frac{39y}{2} to 20y.

$$\frac{79}{2}y+\frac{91}{2}=322$$

Subtract \frac{91}{2} from both sides of the equation.

$$\frac{79}{2}y=\frac{553}{2}$$

Divide both sides of the equation by \frac{79}{2}, which is the same as multiplying both sides by the reciprocal of the fraction.

$$y=7$$

Substitute 7 for y in x=-\frac{3}{2}y-\frac{7}{2}. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-\frac{3}{2}\times 7-\frac{7}{2}$$

Multiply -\frac{3}{2} times 7.

$$x=\frac{-21-7}{2}$$

Add -\frac{7}{2} to -\frac{21}{2} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x=-14$$

The system is now solved.

$$x=-14,y=7$$

Steps Using Matrices

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

$$-2x-3y=7,-13x+20y=322$$

Write the equations in matrix form.

$$\left(\begin{matrix}-2&-3\\-13&20\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}7\\322\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}-2&-3\\-13&20\end{matrix}\right).

$$inverse(\left(\begin{matrix}-2&-3\\-13&20\end{matrix}\right))\left(\begin{matrix}-2&-3\\-13&20\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}-2&-3\\-13&20\end{matrix}\right))\left(\begin{matrix}7\\322\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}-2&-3\\-13&20\end{matrix}\right))\left(\begin{matrix}7\\322\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}-2&-3\\-13&20\end{matrix}\right))\left(\begin{matrix}7\\322\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{20}{-2\times 20-\left(-3\left(-13\right)\right)}&-\frac{-3}{-2\times 20-\left(-3\left(-13\right)\right)}\\-\frac{-13}{-2\times 20-\left(-3\left(-13\right)\right)}&-\frac{2}{-2\times 20-\left(-3\left(-13\right)\right)}\end{matrix}\right)\left(\begin{matrix}7\\322\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{20}{79}&-\frac{3}{79}\\-\frac{13}{79}&\frac{2}{79}\end{matrix}\right)\left(\begin{matrix}7\\322\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{20}{79}\times 7-\frac{3}{79}\times 322\\-\frac{13}{79}\times 7+\frac{2}{79}\times 322\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-14\\7\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=-14,y=7$$

Steps Using Elimination

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.

$$-2x-3y=7,-13x+20y=322$$

To make -2x and -13x equal, multiply all terms on each side of the first equation by -13 and all terms on each side of the second by -2.

$$-13\left(-2\right)x-13\left(-3\right)y=-13\times 7,-2\left(-13\right)x-2\times 20y=-2\times 322$$

Simplify.

$$26x+39y=-91,26x-40y=-644$$

Subtract 26x-40y=-644 from 26x+39y=-91 by subtracting like terms on each side of the equal sign.

$$26x-26x+39y+40y=-91+644$$

Add 26x to -26x. Terms 26x and -26x cancel out, leaving an equation with only one variable that can be solved.

$$39y+40y=-91+644$$

Add 39y to 40y.

$$79y=-91+644$$

Add -91 to 644.

$$79y=553$$

Divide both sides by 79.

$$y=7$$

Substitute 7 for y in -13x+20y=322. Because the resulting equation contains only one variable, you can solve for x directly.

$$-13x+20\times 7=322$$

Multiply 20 times 7.

$$-13x+140=322$$

Subtract 140 from both sides of the equation.

$$-13x=182$$

Divide both sides by -13.

$$x=-14$$

The system is now solved.

$$x=-14,y=7$$