Cant figure out how to calculate the forces, can someone please help?

a boy weighing 4.00 x10^3N jumps from a height of 2.00m to the ground below. Assume that the force of the ground on his feet is constant. (a)compute the force of the ground on his feet if he jumps stiff-legged, the ground compresses 2.00cm, and the compression of the tissue and bones is negligible. (b)compute the force his legs exert on his upper body, which weighs 2.50 x10^3N, under the condition assumed above. (c) now suppose that his knees bend on impact, so that his trunk moves downward 40.0cm during deceleration. compare the force his legs exert on his upper body.