A 5.00 kg ball (ball 1), moving to the right at a velocity of +4.00 m/s on a frictionless table, collides head-on with a stationary 7.60 kg ball (ball 2). Find the final velocities of the balls if the collision is as specified below.

Question

A 5.00 kg ball (ball 1), moving to the right at a velocity of +4.00 m/s on a frictionless table, collides head-on with a stationary 7.60 kg ball (ball 2). Find the final velocities of the balls if the collision is as specified below. 
(a) elastic collision 
ball 1 
ball 2 
(b) completely inelastic collision 
ball 1
ball 2

i solved part a but cant figure out part b. if i make the assumption that in a completley ineleastic equation the balls are stuck togtether and have the same velocity, how would i go about solving it.