A mountain range 4 km high is in Airy isostatic equilibrium. During a period of erosion, a 2 km thickness of material is removed from the mountains. When the new isostatic equilibrium is achieved, how...

A mountain range 4 km high is in Airy isostatic equilibrium. 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?
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/m^3 and 3300 km/m^3, respectively.