Two atomic particles approach each other in a head-on collision. Each particle has a mass of 2.16

Question:

Two atomic particles approach each other in a head-on collision. Each particle has a mass of 2.16 × 10-25 kg. The speed of each particle is 2.10 × 108 m/s when measured by an observer standing in the laboratory.
(a) What is the speed of one particle as seen by the other particle?
(b) Determine the magnitude of the relativistic momentum of one particle, as it would be observed by the other.