Answer
785 MeV.
Work Step by Step
The rest mass of the proton/antiproton is 938.3 MeV.
The proton/antiproton creates a photon with energy $hf = h\frac{c}{\lambda}=2.761\times10^{-10}J=1723 MeV$.
The proton/antiproton must have had KE of 1723 MeV-938.3 MeV = 785 MeV.
