# What equation should I use in this isostasy problem?

I have this problem, and I am not sure where to start. The only Airy equation in my text is $r_1= \frac{h_1 \cdot p_u}{p_s - p_u}$. This doesn't seem to apply, or if it does it isn't obvious. Is there some other equation I should be using?

A mountain range 4 km high is in Airy isostatic equilibrium. (10 points)

(a) During a period of erosion, a 2 km thickness of material is removed from the mountains. When the new isostatic equilibrium is achieved, how high are the mountains?

(b) How much material must be eroded to bring the mountains eventually down to the sea level? Use the crust and mantle densities of 2800 kg/m3 and 3300 km/m3, respectively.

C) How High would they be if 10km of material were eroded away?