A positron moving in the positive x-direction at 2 * 10^8 m/s collides with an electron at rest. The positron and electron annihilate, producing two gamma-ray photons. Photon 1 travels in the positive x-direction and photon 2 travels in the negative x-direction. What is the speed of photon 1 and photon 2? Hint it's not the speed of light.