Answer
$p = 4.9~mc$
Work Step by Step
We can find the magnitude of the momentum:
$E^2 = p^2c^2+m^2c^4$
$(5.0~mc^2)^2 = p^2c^2+m^2c^4$
$25~m^2c^4 = p^2c^2+m^2c^4$
$p^2c^2 = 24~m^2c^4$
$p^2 = 24~m^2c^2$
$p = \sqrt{24}~mc$
$p = 4.9~mc$
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