What is the speed of photon 1? and What is the speed of photon 2?

A positron moving in a positive x-direction at 2x 10^8m/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. Please help me..or at least give me some hints..thank you :)