A wagon is rolling forward on level ground. Friction is negligible. The person sitting in the wagon is holding a rock. The total mass of the wagon, rider, and rock is 94.2 kg. The mass of the rock is 0.323 kg. Initially the wagon is rolling forward at a speed of 0.516 m/s. Then the person throws the rock with a speed of 16.5 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.