Answer
$E = 2.56~MeV$
Work Step by Step
We can find $\gamma$:
$\gamma = \frac{1}{\sqrt{1-\frac{v^2}{c^2}}}$
$\gamma = \frac{1}{\sqrt{1-\frac{(0.600~c)^2}{c^2}}}$
$\gamma = \frac{1}{\sqrt{1-0.360}}$
$\gamma = 1.25$
We can find the mass of the particle:
$E = \gamma~mc^2$
$m = \frac{E}{\gamma~c^2}$
$m = \frac{0.638~MeV}{(1.25)~c^2}$
$m = 0.5104~MeV/c^2$
We can find the new total energy:
$E = \gamma~mc^2$
$E = ( \frac{1}{\sqrt{1-\frac{v^2}{c^2}}})~(mc^2)$
$E = ( \frac{1}{\sqrt{1-\frac{(0.980~c)^2}{c^2}}})~(0.5104~MeV/c^2)(c^2)$
$E = ( \frac{1}{\sqrt{1-(0.980)^2}})~(0.5104~MeV)$
$E = 2.56~MeV$
