Some fusion reactors use an induction process in which thermal neutrons collide with deuterium particles that are essentially twice as massive $($consisting of one proton and one neutron$).$ Consider a scenario in which a neutron is traveling at $2100$ m$/$s when it collides elastically with a deuterium particle that is initially at rest. Find the magnitude of the neutron's recoil velocity $($in m$/$s$).$