Solution: A photon strikes a free electron that is initially at rest. After the photon has scattered from the electron, the photon has wavelength 0.0860 nm and is traveling backwards at an angle of 180° from it

A photon strikes a free electron that is initially at rest. After the photon has scattered from the electron, the photon has wavelength 0.0860 nm and is traveling backwards at an angle of 180° from its original direction. What was the wavelength of the photon before it scattered from the electron? (Express your answer in nm.)