How fast must the tackle be going before the collision for their speed afterward to be zero?

Question

A running back with a mass of 96 kg and a speed of 7 m/s collides with, and is held by, a 144-kg defensive tackle going in the opposite direction. How fast must the tackle be going before the collision for their speed afterward to be zero?

Answer 1

Well by conservation of momentum, momentum afterwards must be zero so total momentum before tackle = 0.

If v is the tackle speed then total momentum before is 96 x 7 - 144 v = 0 giving v = 4.67 m/s