A ball of mass 0.200 kg has a velocity of 150i m/s; a ball of mass 0.300

Question:

A ball of mass 0.200 kg has a velocity of 150i m/s; a ball of mass 0.300 kg has a velocity of ─0.400i m/s. They meet in a head-on elastic collision.
(a) Find their velocities after the collision.
(b) Find the velocity of their center of mass before and after the collision.