Explanation

(b) Using Einstein’s realtion for relativistic mass
$m = \frac{m_{0}}{\sqrt{1 - V^{2}/C^{2}}}\lbrack m_{0} = rest\ mass\rbrack$
$\Rightarrow \frac{m}{m_{0}} = \frac{1}{\sqrt{1 - V^{2}/C^{2}}}$
Given $\frac{m}{m_{0}} = 2 = \frac{1}{\sqrt{1 - V^{2}/C^{2}}}$
$\Rightarrow 1 - \frac{V^{2}}{C^{2}} = \frac{1}{4} \Rightarrow \frac{V^{2}}{C^{2}} = \frac{3}{4}$
$\Rightarrow \frac{V}{C} = \frac{\sqrt{3}}{2} \Rightarrow V = \frac{\sqrt{3}}{2}C$