1. The problem statement, all variables and given/known data 2. Relevant equations E^2=m^2~p^2 k mass = 497.7Mev/c^2 pion mass = 139.6 Mev/c^2 3. The attempt at a solution Using the rest frame i figured that E_pi=m_k/2 so P_pi = sqrt(E_pi^2 - m_pi^2) the pion energy is half of the k meson mass so substituting that i end up with 206Mev/c for each pion. So if momentum is conserved then surely 2*P_pi should be P_k in the frame where the k meson is moving? so using E=sqrt(497.7^2 + 412^2) I got 646Mev for the k meson energy. Is this a valid method where I used the rest frame to find the momenta of the pions first?