Answer
The mass of a negative pion is $139.6\hspace{2mm}MeV/c^2$ = $\large m_\pi$
The speed of light in vacuum = $c$ = $3\times 10^8$ m/s.
The speed of negative pion = $v$ = $2.98\times 10^8$ m/s.
Work Step by Step
Let $p$ is the momentum of the negative pion.
$p=1.19 \hspace{2mm}GeV/c$
Lorentz factor = $\gamma$ = $\Large \frac{1}{\sqrt{1-(\frac{v}{c})^2}}$ where $v$ is the speed of the negative pion.
We also use the conversion: $\large 1$ MeV = $\large 10^{-3}$ GeV.
Thus we have:
