Answer
$r^2=\dfrac{3V}{\pi{h}}$
Work Step by Step
Multiply 3 to both sides:
$3\cdot V=3 \cdot \frac{1}{3}\pi{r^2}h
\\3V=\pi{r^2}h$
Divide by $\pi{h}$ on both sides:
$\dfrac{3V}{\pi{h}} = \dfrac{\pi{r^2}h}{\pi{h}}
\\\dfrac{3V}{\pi{h}} = r^2
\\r^2=\dfrac{3V}{\pi{h}}$
