\left\{ \begin{array} { l } { 8 c - 3 y = 30 } \\ { 5 x - 3 y = 21 } \end{array} \right.
Solve for x, y
x=\frac{8c-9}{5}
y=\frac{8c}{3}-10
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-3y+8c=30,-3y+5x=21
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+8c=30
Pick one of the two equations which is more simple to solve for y by isolating y on the left hand side of the equal sign.
-3y=30-8c
Subtract 8c from both sides of the equation.
y=\frac{8c}{3}-10
Divide both sides by -3.
-3\left(\frac{8c}{3}-10\right)+5x=21
Substitute -10+\frac{8c}{3} for y in the other equation, -3y+5x=21.
30-8c+5x=21
Multiply -3 times -10+\frac{8c}{3}.
5x=8c-9
Subtract 30-8c from both sides of the equation.
x=\frac{8c-9}{5}
Divide both sides by 5.
y=\frac{8c}{3}-10,x=\frac{8c-9}{5}
The system is now solved.
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