momentum can be calculated using the following rule:
momentum = mass x velocity
For this proton we have:
mass of proton is a constant = 1.67 x 10^-27 kg
velocity = 0.772 c where c is the constant speed of light
velocity = 0.772 x 3 x 10^8 = 231.6 x 10^6 m/sec
Substitute with the mass and velocity to calculate the momentum as follows:
momentum = 1.67 x 10^-27 x 231.6 x 10^6 = 3.86772 x 10^-19 kg.m/sec