A balloon rises at at constant velocity of 3m/s. A rock is released over the side.5 seconds later this rock hits the ground.

A balloon rises at at constant velocity of 3m/s. A rock is released over the side. 4.5 seconds later this rock hits the ground. Calculate the height of the rock when it is halfway between the balloon and the ground. Assume Earth gravity (-9.81m/s), neglect air resistance.