Question

Estimate the X-ray energy for anti-proton/proton annihilation

Answer 1

What are the rest energies of protons and antiprotons?

Answer 2

The proton has a rest mass of 938 MeV, so if all of the energy went into the photon, you'd have 2 x 938 = 1.876 GeV. Incidentally, that's not an X-ray, that's a very energetic gamma ray.