Solve for x
x=\frac{y-21}{3}
Solve for y
y=3\left(x+7\right)
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y=3x+21
Use the distributive property to multiply 3 by x+7.
3x+21=y
Swap sides so that all variable terms are on the left hand side.
3x=y-21
Subtract 21 from both sides.
\frac{3x}{3}=\frac{y-21}{3}
Divide both sides by 3.
x=\frac{y-21}{3}
Dividing by 3 undoes the multiplication by 3.
x=\frac{y}{3}-7
Divide y-21 by 3.
y=3x+21
Use the distributive property to multiply 3 by x+7.
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