Answer
$l = \frac{P-2w}{2}$
Work Step by Step
We want to solve for $l$:
First, let's distribute what is in the parentheses:
$P = 2l + 2w$
Subtract $2w$ from each side to isolate the $l$ term on one side of the equation:
$P - 2w=2l$
Divide both sides of the equation by $2$ to solve for $l$:
$\frac{P-2w}{2}=l$