Answer
$2.83\times10^{8}\,m/s$
Work Step by Step
$\text{Relativistic momentum}=\frac{mv}{\sqrt {1-\frac{v^{2}}{c^{2}}}}$
Given $\frac{mv}{\sqrt {1-\frac{v^{2}}{c^{2}}}}=3mv$
$\implies \frac{1}{3}=\sqrt {1-\frac{v^{2}}{c^{2}}}$ or $\frac{1}{9}=1-\frac{v^{2}}{c^{2}}$
$\implies \frac{v^{2}}{c^{2}}=1-\frac{1}{9}=\frac{8}{9}$
$\implies v=\sqrt {\frac{8}{9}}c=0.9428\times3.00\times10^{8}\,m/s$
$=2.83\times10^{8}\,m/s$
