Question

A 0.060 kg tennis ball, moving with a speed of 7.00 m/s has a head-on collision with a 0.086 kg ball initially moving in the same direction at a speed of 3.10 m/s. Assume a perfectly elastic collision and take the initial direction of the balls as positive.

Answer 1

you have two equations:
first, conservation of momentum. Solve for one ball's final velocity in terms of the other.
then write conservation of eneryg. Substiture your first velocity, then expand it,and solve. A bit of algebra is required.