satelite in circular orbit?

A satellite moves in a circle in the earth’s equatorial plane. The satellite is at such a height above the surface of the earth that it appears stationary or not moving (such an
orbit is called geosynchronous).

a. Find the altitude or height of this satellite above the surface of the earth.

b. Explain with a sketch why the radio signals from these satellites cannot directly
reach receivers on earth that are north or south of specific latitude.