A photon having 20 keV scatters from a free electron at rest. What is the maximum energy that the electron can obtain? 2. Relevant equations [itex]\lambda[/itex]' - [itex]\lambda[/itex] = (h/mc)(1-cos(theta) 3. The attempt at a solution i just dont know where to get started or anythign can someone show me how to do this?