In your set up, Please show how to cancel out or move the variables from one side to the other.

A mass 3 m collides head-on with a mass m. Before the collision the masses are moving directly toward one another with the same speed vo. vo 3 m After the collision, the mass 3 m is moving in its original direction at speed cold 3 U 3 m What is the speed v of the mass m after the collision