$\begin{align}  & \text{Surface area of an hemisphere}=2\pi {{r}^{2}} \\ & \text{Volume of an hemisphere is }\frac{2}{3}\pi {{r}^{3}} \\ & \frac{2}{3}\pi {{r}^{3}}=2\pi {{r}^{2}} \\ & r=3cm \\\end{align}$