a) Substitute t = 4 and y = 20 into y = gt^2. Solve for g.

b) Average speed = distance/time = .....

c) t = 4 s, s = 20 m, u = 0 m/s, v = ?

Substitute the data into s = (u + v)t/2 (one of the usual straight line motion under constant acceleration formula).

Alternatively, t = 4, s = 20, a = g = answer to (a), v = ?

Substitute the data into s = vt - at^2/2 (one of the usual straight line motion under constant acceleration formula).