1. The problem statement, all variables and given/known data 2. Relevant equations de Broglie wavelength λ = h/p E^2 = p^2 c^2 + (m0)^2 c^4 L-compton = (h-bar) / mc 3. The attempt at a solution I'm trying to work out the wavelength of an electron with a kinetic energy of 1keV So I intend on using de Broglie wavelength λ = h/p and Relativistic mass equation E^2 = p^2 c^2 + (m0)^2 c^4 then inputting the rest mass energy of an electron (511 keV) into the formula above. Though do I need to convert this first to kg? Also, I'd still be left with two unknowns in mass eqn i.e. E and p, so how do I obtain the value for E? Or should I instead be using the Compton wavelength formula i.e. L-compton = (h-bar) / mc The question just asks for the wavelength.