What is the maximum frequency photon of one of the photons produced when a electron and positron annihilate each other?

First we must remember that a positron is the antiparticle of the electron, meaning that they will both have the same amount of rest energy (0.510999 MeV) however since annihilation produces 2 gamma photons each photon will have the same rest energy as one of the particles (0.510999 MeV). We then use the equation E=hc/λ rearranged to λ=hc/E and then sub in:(6.63x10^-34)(3x10⁸)/(0.511x10⁶)(1.60x10^-19) Notice here that in the denominator i have converted (0.510999 MeV) into MeV and then multiplied by the charge of an electron to get the energy in Joules (J) .
This then gives us the maximum wavelength of a single photon during a photon-positron annihilation: 2.43x10^-12m