A moving proton of kinetic energy 2.12 GeV collides with a stationary antiproton. These particles annihilate and a new particle of rest mass 2.74 GeV/c² and momentum 2.91 GeV/c is produced. Show that no other new particles have been produced as a result of this collision.
(rest mass of proton = GeV/c².)
I had a bash at it, but I'm sure I've gone wrong. I think you have to use relativistic formulae.