How do I solve this?

In an electron-positron collider, the electron is accelerated to an energy of 5.11 MeV. The electron mass is 511 keV/c2. (a) What is the velocity (expressed as v/c), kinetic energy, and momentum of the electron? A positron is accelerated in the opposite direction to the same energy and collides with the electron. (b) What is the maximum mass of a new particle X created by the collision [ette- -> X] ? What is the new particle's momentum in this case? (c) What is the velocity (expressed as v/c) of the positron in the electron's rest frame? [Careful! This calculation requires some precision.] (d) What is the total energy in this frame? Discuss why most accelerators now use colliding beams as opposed to fixed targets. A new particle, the Techion, is created with a rest mass 3 MeV and total energy 6 MeV in the laboratory frame. In the lab frame, the Techion travels 2 m between where it is created and where it decays. (c) What is the proper decay time (i.c., the decay time in its own rest frame) for the Techion