Volume of the cone V = 1/3*π*r^2*h.....(1)

You want to find dh/dt. So you have to find r in terms of h.

r and h are proportional tp R and H, where R = 21" and H = 15".

So R/H = r/h.

So r = (R/H)*h.

Substitute this value in the eq.(1) and find dV/dt.

dV/dt is given. Find dh/dt.