Answer
$r = \sqrt{\dfrac{3V}{\pi{h}}}$
Work Step by Step
Multiply $3$ to both sides to obtain:
$3V=\pi{r^2h}$
Divide $\pi{h}$ to both sides to obtain:
$\dfrac{3V}{\pi{h}}=r^2$
Take the square root of both sides to obtain:
$\pm\sqrt{\dfrac{3V}{\pi{h}}}=r$
The $r$ in this formula/equation is radius. A radius is a length therefore it cannot be negative.
Thus,
$r = \sqrt{\dfrac{3V}{\pi{h}}}$
