Transpose for h. V = (h/6)(3r2 + h2) make h subject of the formula...
Is that V=(h/6)(3r2+h2) ?
If so you need to solve a cubic to make h the subject. There are formulae, which you can Google if necessary, but I wonder what the whole question is that makes this a necessity.
Edit: I see you've raised the exact same question before!