Finding smallest dimension through optimisation

What is the smallest dimension of a cone to get a volume of $\displaystyle 48 \pi $?

From the cone formula, I get that $\displaystyle r^2 h = 144$. I know it is an optimisation problem but I couldn't figure out what is the equation that I need to form to get its derivative equals to zero.

I tried to make r in terms of h by having it as $\displaystyle (\frac{12}{\sqrt{h}})^2 \times h = 144$ but it will all cancel out to just 144=144 before I could differentiate anything. (Thinking)