During an Apollo lunar landing mission, the command module continued to orbit the Moon at an altitude of about 126km. How long did it take to go around the Moon once?

It will take the command module 7159 seconds, or about 2 hours to go around the moon.

Explanation:

We use Kepler's third law which says the period of an orbit is

where is the distance from the center of the moon, is the gravitational constant, and is the mass of the moon.

Now,

putting these into Kepler's law, we get:

which is 119 minutes or 1 hour 59 minutes, which is approximately 2 hours.