1. The problem statement, all variables and given/known data A 0.7MeV (E°) photon scatters off a free electron such that the scattering angle of the photon is twice the scattering angle of the electron (θ=2φ). Determine the scattering angle for the electron. 2. Relevant equations λ' - λ° = h/(mec)(1-cosθ) E = hc/λ ρelectron = mev / √(1-v2/c2) 3. The attempt at a solution The initial wavelength of the photon (λ°) = hc/E° = 5.92x10-13m Honestly, I'm not sure at all where to go from here. I know it's a conservation of momentum problem, but based on the equation above, I need the velocity of the electron in order to find it's momentum.