Answer
$l=\dfrac{A-2wh}{2w+2h}$
Work Step by Step
Subtract $2wh$ on both sides to find:
$A-2wh=2lw+2lh$
Factor out $l$ on the right side to find:
$A-2wh=l(2w+2h)$
Divide by $2w+2h$ on both sides to find:
$\dfrac{A-2wh}{2w+2h} = l
\\l=\dfrac{A-2wh}{2w+2h}$