Question 13423
You could try this:

{{{S = 2(pi)rh + 2(pi)r^2}}} Factor {{{2(pi)}}} and divide both sides by it.
{{{S/2(pi) = rh + r^2}}} Subtract {{{S/2(pi)}}} from both sides.
{{{r^2 + hr - S/2(pi) = 0}}} Solve for r using the quadratic formula.
{{{r = (-h+-sqrt((-h)^2 - 4(-S/2(pi))))/2}}}
{{{r = (-h+-sqrt(h^2+(2S/(pi))))/2}}}