The nucleus of 214Po decays radioactively by emitting an alpha particle (mass 6.65*10^-27) with kinetic energy 1.23*10^-12 J, as measured in the laboratory reference frame. Assuming that the Po was initially at rest in this frame, find the magnitude of the recoil velocity of the nucleus that remains after the decay. I used the kinetic energy to solve for the velocity: k=.5mv^2 which is 19233429 m/s. Now I'm not sure what to do with that. I think I'm suppose to use the conservation of momentum theory, but I don't know how.