Answer
798.7 MeV
Work Step by Step
There is no initial momentum and no final momentum. The pions have the same speed, and therefore the same kinetic energy.
Use energy conservation to find the KE of a pion.
$$2m_pc^2=2KE_{\pi}+2m_{\pi}c^2$$
$$ KE_{\pi}=m_pc^2-m_{\pi}c^2=938.3MeV-139.6MeV$$
$$ KE_{\pi}=798.7MeV$$
