Answer
$p = 1.0\times 10^{-18}~kg~m/s$
Work Step by Step
We can find the momentum of the proton:
$p = \gamma~m~v$
$p = \frac{1}{\sqrt{1-\frac{v^2}{c^2}}}~m~v$
$p = \frac{1}{\sqrt{1-\frac{(0.90~c)^2}{c^2}}}~(1.67\times 10^{-27}~kg)~(0.90~c)$
$p = \frac{1}{\sqrt{1-(0.90)^2}}~(1.67\times 10^{-27}~kg)~(0.90)(3.0\times 10^8~m/s)$
$p = 1.0\times 10^{-18}~kg~m/s$
