Answer
$p=2.83mc$
Work Step by Step
It is given that the total energy of the particle is 3.00 times its rest energy.
Therefore;
$\gamma=\frac{E}{E_{\circ}}=3.00$
We know that
$\gamma=\frac{1}{\sqrt{1-(\frac{v}{c})^2}}$
$\implies 3.00=\frac{1}{\sqrt{1-(\frac{v}{c})^2}}$
After simplification, we obtain:
$v=0.94281c$
We also know that:
$p=\gamma m v$
We plug in the known values to obtain:
$p=3.00\times m\times (0.94281c)$
$p=2.83mc$
