Answer:
It must return at the same velocity with which it was thrown or else there would be a preferred direction of motion leading to perpetual motion.
a = (v2 - v1) / t = (v2 - 0) / 2 = v2 / 2 for a falling object
v2 = 2 a = 2 sec * 9.8 m/sec^2 = 19.6 m/s
You can check with
H = V0 t - 1/2 g t^2 (V0 and g are in different directions)
H = 0
V0 = 1/2 g * 4 = 19.6 m/s
