Two particles of masses M and 3M are moving towards each other along the +x and -x directions with the same initial speed 3.29 m/s. They undergo a 'head-on' elastic collision and each rebounds along the same line as it approached. What is the final speed of the particle of mass M in m/s? so i used mv + mv = mv + mv so 3.29M -3.29(3M) = M(v) + 3M(v): -6.58 = v + 3v v = -1.645 now im confused, i know this isn't the right answer. I don't know what to do next. thanks.