Solve {r}{-6x+21y=-24}{+quad6x-4y=24}

$$\left. \begin{array} { r } { - 6 x + 21 y = - 24 } \\ { + \quad 6 x - 4 y = 24 } \end{array} \right.$$

Solve for x, y

$$x=4$$

$$y=0$$

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.

$$-6x+21y=-24,6x-4y=24$$

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

$$-6x+21y=-24$$

Subtract 21y from both sides of the equation.

$$-6x=-21y-24$$

Divide both sides by -6.

$$x=-\frac{1}{6}\left(-21y-24\right)$$

Multiply -\frac{1}{6} times -21y-24.

$$x=\frac{7}{2}y+4$$

Substitute \frac{7y}{2}+4 for x in the other equation, 6x-4y=24.

$$6\left(\frac{7}{2}y+4\right)-4y=24$$

Multiply 6 times \frac{7y}{2}+4.

$$21y+24-4y=24$$

Add 21y to -4y.

$$17y+24=24$$

Subtract 24 from both sides of the equation.

$$17y=0$$

Divide both sides by 17.

$$y=0$$

Substitute 0 for y in x=\frac{7}{2}y+4. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=4$$

The system is now solved.

$$x=4,y=0$$

Steps Using Matrices

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

$$-6x+21y=-24,6x-4y=24$$

Write the equations in matrix form.

$$\left(\begin{matrix}-6&21\\6&-4\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-24\\24\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}-6&21\\6&-4\end{matrix}\right).

$$inverse(\left(\begin{matrix}-6&21\\6&-4\end{matrix}\right))\left(\begin{matrix}-6&21\\6&-4\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}-6&21\\6&-4\end{matrix}\right))\left(\begin{matrix}-24\\24\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}-6&21\\6&-4\end{matrix}\right))\left(\begin{matrix}-24\\24\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}-6&21\\6&-4\end{matrix}\right))\left(\begin{matrix}-24\\24\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{4}{-6\left(-4\right)-21\times 6}&-\frac{21}{-6\left(-4\right)-21\times 6}\\-\frac{6}{-6\left(-4\right)-21\times 6}&-\frac{6}{-6\left(-4\right)-21\times 6}\end{matrix}\right)\left(\begin{matrix}-24\\24\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{2}{51}&\frac{7}{34}\\\frac{1}{17}&\frac{1}{17}\end{matrix}\right)\left(\begin{matrix}-24\\24\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{2}{51}\left(-24\right)+\frac{7}{34}\times 24\\\frac{1}{17}\left(-24\right)+\frac{1}{17}\times 24\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}4\\0\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=4,y=0$$

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.

$$-6x+21y=-24,6x-4y=24$$

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

$$6\left(-6\right)x+6\times 21y=6\left(-24\right),-6\times 6x-6\left(-4\right)y=-6\times 24$$

Simplify.

$$-36x+126y=-144,-36x+24y=-144$$

Subtract -36x+24y=-144 from -36x+126y=-144 by subtracting like terms on each side of the equal sign.

$$-36x+36x+126y-24y=-144+144$$

Add -36x to 36x. Terms -36x and 36x cancel out, leaving an equation with only one variable that can be solved.

$$126y-24y=-144+144$$

Add 126y to -24y.

$$102y=-144+144$$

Add -144 to 144.

$$102y=0$$

Divide both sides by 102.

$$y=0$$

Substitute 0 for y in 6x-4y=24. Because the resulting equation contains only one variable, you can solve for x directly.

$$6x=24$$

Divide both sides by 6.

$$x=4$$

The system is now solved.

$$x=4,y=0$$