Answer :
[tex]V=\frac{1}{3}\pi r^2h\\\\\frac{1}{3}\pi r^2h=V\ \ \ \ |multiply\ both\ sides\ by\ 3\\\\\pi r^2h=3V\ \ \ \ |divide\ both\ sides\ by\ \pi h\\\\r^2=\frac{3V}{\pi h}\Rightarrow\boxed{r=\sqrt{\frac{3V}{\pi h}}}[/tex]
