Answer
$1.3\frac{MeV}{c^2}=2.3\times10^{-30}kg$
Work Step by Step
A photon has momentum $0.65\frac{MeV}{c}$, and therefore each has energy of 0.65 MeV. Let the photons make a head-on collision so that the system momentum is 0.
The created particle will be stationary, so all of the initial energy (of the 2 photons) turns into the mass-energy of the new particle.
The heaviest possible particle has a mass of $1.3\frac{MeV}{c^2}$. (In kg, this is $2.3\times10^{-30}kg$).
