Can anyone help me with this problem? Two billiard balls of equal mass undergo a perfectly elastic head-on collision. If one ball (ball A) is initially moving at 1.9 m/s in the +x direction, and the other (ball B) is initially moving at 3.9 m/s in the -x direction, what will be the velocities of (a) ball A and (b) ball B after the collision? (Indicate the direction by the sign of your answer, taking a positive velocity as being in the +x direction.) I've tried using the conservation of momentum and energy... doesn't seem to work.