1) What is the volume of a cone?

2) Since the diameter is twice the radius and the diameter is twice the height, in what way can we relate the radius of the conical tank and the height of the conical tank?

If you can answer both of these questions, you will be able to come up with a volume equation that is dependent only on the height.

From that point, differentiate V with respect to h and then evaluate it when cubic feet/min and when feet.

Does this make sense? Can you take it from here?