L$11-1[20$ marks$]$ An electron of mass $\backslash ($ m $\backslash )$ carries momentum $\backslash ($ p$\_\{0\}\backslash ).$ It travels directly towards a positron $($of the same mass$)$ at rest. They annihilate after collision, producing two photons A and B$.$ Assume the photon A travels in a direction forming an angle of $60$ degrees with respect to the traveling direction of the electron. Determine the energy of the photon A$.$