Solve for a
a=\frac{P-2l}{2}
Solve for P
P=2\left(a+l\right)
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2a+2l=P
Swap sides so that all variable terms are on the left hand side.
2a=P-2l
Subtract 2l from both sides.
\frac{2a}{2}=\frac{P-2l}{2}
Divide both sides by 2.
a=\frac{P-2l}{2}
Dividing by 2 undoes the multiplication by 2.
a=\frac{P}{2}-l
Divide P-2l by 2.
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