Answer
$2.6\times10^8 m/s$.
Work Step by Step
The problem asks for a speed such that the value of $\gamma$ is $\frac{1}{\sqrt{1-u^{2}/c^{2}}}=2$.
$u=c\sqrt{1-0.5^2}=(3.00\times10^8m/s)\sqrt{0.75}=2.6\times10^8 m/s $
This is far, far faster than any speed we can currently achieve in a jet.
