This is the question:

A hollow right circular cone has height 18 cm and base radius 12 cm.

It is held vertex downwards beneath a tap leaking at the rate of 2 cm/s.

Find the rate of rise of water level when the depth is 6 cm.

I have no idea where to go with this question. This is what i've tried:

Then i can find out the volume of the cone easily in respect to either r or h but i don't understand how i can use this in order to find the change in the height of the depth of the water.

Can someone please explain how i can proceed to complete this question. Thanks in advance.