So I'm looking at my notes, and there's a section on annihilation. It explains that in a particle/ anti-particle collision, momentum is conserved. In the problem, an electron with velocity= 8 x 10^6 m/s collides with a positron with velocity 3 x 10^6 m/s, producing two photons. The equation was set up like this 2mc² + Ek + Ek = 2hf Except instead of keeping the Ek's separate for the electron and the positron, they put them together as one Ek, adding the velocities and the masses. Why would you do this?