The diameter of the earth at the equator is appoximately 7900 what is the appoximate surface area of the earth?

From mathematics, if a sphere has a given radius R then the surface area is A=4*pi*R2

So: R=7900/2=3950

A=4*pi*39502=196,066,798 km2

In reality it is 510,000,000 km2 because Earth is not an exact sphere and its radius is 6,378.15 km.