A horse is running 35 m/s when it (mass A 1000kg) runs into a pig (Mass B= 350 kg) that is standing still (V=0). If the horse is running at 20 m/s (Va2) how fast is the pig traveling at. (Vb2).

Assuming Conservation of Momentum, we have that total momentum must be equal before and after the collision.

Momentum for a body is given by p = mv; where p is momentum, m is mass and v is velocity. Technically p and v are vectors with directions, but in this simple example we can assume the horse and pig move in the same direction, so we don't need to overcomplicate things!