Answer
$p = 1.45~MeV/c$
Work Step by Step
$mc^2 = 0.511~MeV$
$mc = 0.511~MeV/c$
Since the total energy is 3.00 times the rest energy, the total energy $E = 3.00~mc^2$
We can find the momentum $p$:
$E^2 = p^2c^2+m^2c^4$
$(3.00~mc^2)^2 = p^2c^2+m^2c^4$
$9.00~m^2c^4 = p^2c^2+m^2c^4$
$p^2c^2 = 8.00~m^2c^4$
$p^2 = 8.00~m^2c^2$
$p = \sqrt{8.00}~mc$
$p = (\sqrt{8.00})~(0.511~MeV/c)$
$p = 1.45~MeV/c$
