Contextual Applications of Differentiation · Superchallenge problem

Water drains from a conical tank (vertex down) whose radius is always half its depth, at a rate $\dfrac{dV}{dt}=-5\sqrt h$ cubic feet per minute (Torricelli's law), where $h$ is the depth in feet. Find $\dfrac{dh}{dt}$, in ft/min, at the instant $h=9$ feet.