Question

What is a equals 21 W + 21 h +2wh solve for h

Answer 1

If you mean: a= 21w + 21h + 2wh

then

you subtract 21w and 21h from both side.

here will look like: a - 21w - 21h= 2wh

then divide 2w from each side

look like this: a - 21w - 21h / 2w = 2w/h

answer is  a - 21w - 21h / 2w = h