Physics of an isochronous satellite?

An isochronous satellite of Earth is one that stays above the same point on the equator of Earth. Using Kepler's Third Law and Earth's moon as a reference, determine the height above Earth's surface for such a satellite, Do not actually calculate the height in metres. Give your answer in terms of the number of Earth radii. Also, do not use the period of motion of the moon in seconds. Use the fact that the moon's period is 27 days, 27d.