Answer
$5.4 \times 10^4 km/h$.
Work Step by Step
The formula for Galilean relative velocities is that $$v_{rel.}=u-v$$ where $u$ is the observer's velocity and $v$ is the observed object's speed. Since the other satellite is moving opposite of the observer, its velocity is negative. This means that the relative velocity is $$v_{rel.}=(2.7 \times 10^4km/h)-(-2.7 \times 10^4 km/h)$$ $$=5.4 \times 10^4km/h$$
