I was working on the following problem: An electron and a positron each have a mass of 9.11 x 10-31 kg. They collide and both vanish, with only electromagnetic radiation appearing after the collision. If each particle is moving at a speed of 0.30c relative to the laboratory before the collision, determine the energy of the electromagnetic radiation. I feel like I'm missing something basic here. I know the formula is E=mc2. However my webassign says my answer is wrong. Am I wrong to assume that the mass is 2*9.11e-31? Also don't I just multiply 0.30*c2?