1. The problem statement, all variables and given/known data 2. Relevant equations I have thought about using the momentum formulas here. 3. The attempt at a solution As the momentum is a conservative quantity, I have thought that p (before collision) = p(after collision). Since p (before collision) = 12, I wanted to use that result to find p (after collision) and then find the speed of the ball B. So p(after collision) = ma x v + mb x v = 12 So 12 = v ( 2 + mb) However I have two unknown variables here, so I can't find the speed and obviously, the mass of the ball B.