1. The problem statement, all variables and given/known data 2. Relevant equations conservation of momentum, m1v1i+m2v2i=m1v1f+m2v2f and conservation of KE, ½m1v1i^2+½m2v2i^2=½m1v1f^2+½m2v2f^2 3. The attempt at a solution first, i defined my variables. v1i=-v2i, v1f=-0.750v1i, v1f=0.750v2i I tried isolating V2f using the conservation of momentum equation, getting V2f=[-m1v2i+m2v2i-m1(0.750v2i)]/m2. Then I use the conservation of kinetic energy equation and plug in my new v2f value. expand it out, then try to simply everything to get an answer for m2. I'm really bad at long and complicated algebra, I often cancel out items I'm not allowed to, and don't cancel out when I should. I've tried around 7-8 times now, all with varying answers, none of them right.