Solve s=2(lw+lh+wh) for w
s = 2(lw + lh + wh)Divide each side by 2 : s/2 = lw + lh + whSubtract 'lh' from each side: s/2 - lh = lw + whCombine the 'w' terms: s/2 - lh = w(l + h)Divide each side by (l + h): (s/2 - lh) / (l + h) = w