The speed of light is approximately 3.00 x 10^8 m/s. Then the next step is to convert m/s into km/s.To determine the time, the distance must be divided with velocity. It will take the light signal 3.3*10^-6 seconds to reach his assistant.
The answer is 20km because you go to yours friend's house and back then an extra 10km. 5+5+10=20km
he meant that it was different from most galaxies as it is a "late stage major merger: two recently combined massive galaxies"