please help me on the following problem:

A rock released from rest to fall on a small airless planet falls y=gt^2 m in t sec, g a constant. Suppose that the rock falls to the bottomof a crevasse 20 m below and reaches the bottom in 4 sec.

a) find the value of g.

b) find the average speed of the fall.

c) With what speed did the rock hit the bottom?