Satellite motion — UPSC CSE 1993 Physics PYQ

A satellite, revolving in a circular equatorial orbit at height 1.36 \times 10^4\text{ km} from earth's surface from west to east, appears over some spot at the equator every 6\text{ hrs}. Show that the data are consistent with known earth's radius 6.4 \times 10^3\text{ km} and g = 9.8\text{ ms}^{-2} at earth's surface.