When they are far apart, the momentum of a proton is <3.9 × 10-21, 0, 0> kg·m/s as it approaches another proton that is initially at rest. The two protons repel each other electrically, without coming close enough to touch. When they are once again far apart, one of the protons now has momentum <1.0 × 10-21, 4.6 × 10-21, 0> kg·m/s. At that instant, what is the momentum of the other proton? (Express your answer in vector form.)