Area of the curved surface.
Solve  for r and plug in this term into :
Now differentiate a wrt h (V is a constant!). You'll get:
Solve a'(h) = 0 for h. This is obviously only possible if
Solve for h. Substitute V by the term of  and solve for h again. You'll get
Remark: Especially the differentiation of a is really horrible. So I hope for you that someone knows a simpler, shorter way to solve the question.