Hey there!!
Equation given :
S = 2hl + 2hw + 2lw
... S = 2 ( hl + hw + lw )
S / 2 = hl + hw + lw
S / 2 = h ( l + w ) + lw
S / 2 - lw = h ( l + w )
S - 2lw / 2 = h ( l + w )
( S - 2lw / 2 ) / ( l + w ) = h
( S - 2lw / 2l + 2w ) = h
[tex]\frac{S-2lw}{2l+2w} = h[/tex]
Hope my answer helps!!