1. The problem statement, all variables and given/known data An x-ray photon of wavelength 0.025 nm collides elastically with an electron and scatters through an angle of 90 degrees. How much energy did the electron acquire in this collision and in what way did the x-ray change? 2. Relevant equations E=pc E=hc/lambda 3. The attempt at a solution E=hc/lambda to get energy, which didn't work (6.63e-34)(3.0e8)/(2.5e-11) = 7.96e-15 J Then I read the whole chapter on it and could not find out how, exactly, to relate the two together, thus allowing me to figure out individual energy for the electron. The answer is 7.0e-16, which is pretty irrelevant because I want to know how to do it. Thanks for any input.