I mentioned that the momentums of the balls are opposite i.e they have the same magnitude and opposite directions. I didn't mention that the momentums are opposite INITIALLY, my bad. Anyway, in all my efforts I have made use of the following relationships: m1v1 - m2v2= -m1v1' + m2v2' and KE1 + KE2=KE1' + KE2'. I also used p1=p2 and p1'=p2'. My goal is basically to prove that V1=v1'. I used the above relationships in many different ways, solving and substituting but that doesn't lead me anywhere, only to relationships I know to be true. Any ideas?