Answer:
l = (S - 2wh)/(2w + 2h)
Step-by-step explanation:
S = 2lw + 2wh + 2hl
S - 2wh = 2lw + 2hl
S - 2wh = l (2w + 2h)
(S - 2wh)/(2w + 2h) = l