How to calculate the energy in gamma photons

[Amanuel]

how do I calculate the energy of to photons moving in the opposite direction. starting from the same origin.

 

[berkeman]

E = hγ ?

 

[Amanuel]

I mean using just the mass invarians of relativity

 

[jbriggs444]

I mean using just the mass invarians of relativity

If you have no data to use as input, a formula will not help. What do you know about these photons or about the process that produced them?