Answer
See explanation.
Work Step by Step
The two photons have equal and opposite momentum vectors.
Since E =pc, the photons have equal energies. Each has half the rest energy of the neutral pion.
The rest mass of the pion is 270(0.511 MeV) = 138 MeV.
Each photon has energy of 69 MeV.
Frequency $f=\frac{E}{h}=1.7\times10^{22}Hz$.
Wavelength $\lambda=\frac{c}
{f}=1.8\times10^{-14}m$.
These photons are in the gamma ray part of the EM spectrum.
