S = 2pi(rh) + 2pi (r^2)
subtract 2pi(r^2) from both sides
S - 2pi(r^2) = 2pi(rh)
since the right is all multiplication, you can divide by all pieces at the same time. so divide both sides by 2pi(r)
then h = S/[2pi(r)] - [2pi(r^2)]/[2pi(r)]
simplifying gives h = S/[2pi(r)] - r