Two physicists are doing an experiment using subatomic particles. They know that collisions between subatomic particles are considered to be elastic. During the experiment, they speed up a proton through a magnetic field to a speed of $2\mathrm{.}3$ x $10^8$ m$/$s and smash it into a stationary unknown subatomic particle. If the proton continues east at a velocity of $2\mathrm{.}1$ x $10^8$ m$/$s after the collision, how much energy must the unknown particle have gained?