Problem 3.6: Energy of the photon that hits an electron and produces an electron-positron pair?

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A photon hits an electron at rest as shown in the animation. Restart.  This generates a positron-electron pair which moves off to the right with the same speed as the electron. A positron has the same rest mass as an electron, but is positively charged. Electron (and positron) rest mass is 0.511 MeV/c2. What is the momentum of each of the end products?  What was the energy of initial photon?