The height (in m) that an object has fallen from a height of 180 m is given by the position function s(t) = -5tsquared + 180, where t > or equal to 0 and t is in seconds.

a. Find the average velocity during each of the first 2 seconds.

b. Find the velocity (instantaneous) of the object at t = 4

c. At what velocity will the object hit the ground?

I tried these, and got different answers then the correct ones. Help me out?