I was wondering if someone could help me with a problem I've been struggling with for a bit. I'm just not quite equipped with the concepts to solve it. Two electrons have a relative speed of 0.9c. Calculate the momentum of each electron in the center-of-mass frame, the frame where they have equal and opposite momenta. I don't really have an attempt at a solution yet, because I just don't know how to start. If someone could set me in the right direction, it would be greatly appreciated.