v=1/3pi r^2 h
$\displaystyle V=\tfrac{1}{3}\pi r^2h$
To solve for h, let us first multiply both sides by 3 to get
$\displaystyle 3V=\pi r^2 h$
Now divide by $\displaystyle \pi$ to get
$\displaystyle \frac{3V}{\pi}=r^2 h$
Now to isolate h, divide by $\displaystyle r^2$, and we get what we are looking for:
$\displaystyle \color{red}\boxed{h=\frac{3V}{\pi r^2}}$
Does this make sense?
--Chris