A new model for Dark Matter, suggested by recent observations which see an excess of positrons coming from a halo around the galactic core, consists of a "dark photon"* which can decay into electrons or other, light, particles. Unlike a normal photon dark photons would have a non-zero mass.

One such Dark Photon decays, but not at rest, inside the ATLAS detector and produces an electron travelling along the x axis with a total energy of 1.1 GeV and a positron travelling along the y axis with a total energy of 2.0 GeV. Neglecting the masses of the electron and positron, what is the mass of the Dark Photon which produced them?

[Specify units of 'GeV/c2' or 'MeV/c2' in your answer.]

what I did:
well we never covered this in class and have no examples for it
but what I did is assumed a conservation of Relativistic kinetic energy

Ek = Kef-kif
= 2.0 GeV - 1.1Gev
= .9 GeV

this is incorrect, I don't know how else to do it
since are only given the x and y energies