Answer
$E_{photon}=0.511MeV$
$p_{photon}=0.511MeV/c$
Work Step by Step
To three significant figures, the particles are moving so slowly that their KE is negligible compared to their rest energy.
Assume the total energy equals the rest energy. Each particle had the same rest energy of 0.511 MeV. The photons split the energy, so each has energy of 0.511 MeV.
$$E_{photon}=0.511MeV$$
By equation 27-6, each photon’s momentum is E/c.
$$p_{photon}=0.511MeV/c$$
