The principle of conservation of linera momentum...need help!!!?

A wagon is rolling forward on level ground. Friction is negligible. The person sitting in the wagon is holding a rock. The total m of the wagon, rider, and rock is 92 kg. The m of the rock is 0.28 kg. Initially the wagon is rolling forward at a speed of 0.51 m/s. Then the person throws the rock with a speed of 17 m/s. Both speeds are relative to the ground. Find the speed of the wagon after the rock is thrown (a) directly forward in one case and (b) directly backward in another.