Answer
$p = 6.48~MeV/c$
Work Step by Step
Mote that the rest energy of an electron is: $mc^2 = 0.511~MeV$
We can find the momentum $p$:
$E^2 = (pc)^2+(mc^2)^2$
$(pc)^2 = (6.5~MeV)^2 -(0.511~MeV)^2$
$(pc)^2 = 42.0~(MeV)^2$
$pc = \sqrt{42.0~(MeV)^2}$
$pc = 6.48~MeV$
$p = 6.48~MeV/c$
