Let a particle of charge e and rest mass m, initially at rest in the laboratory; scatter

Question:

Let a particle of charge e and rest mass m, initially at rest in the laboratory; scatter a photon of initial frequency νi. This is called Compton scattering. Suppose the scattered photon comes off at an angle θ from the incident direction. Use conservation of four-momentum to deduce that the photon's final frequency νf is given by
1/vf = 1/vi + h(1-cosθ/m)