Answer
$E = -6.34\times 10^9~J$
Work Step by Step
We can find the total mechanical energy before the collision:
$E = E_A+E_B$
$E = -\frac{GMm}{2r}-\frac{GMm}{2r}$
$E = -\frac{GMm}{r}$
$E = -\frac{(6.67\times 10^{-11}~N~m^2/kg^2)(5.98\times 10^{24}~kg)(125~kg)}{7.87\times 10^6~m}$
$E = -6.34\times 10^9~J$
