Physics
posted by vv .
Earth's mean radius and mass are 6,371 km and 5.97 x 1024 kg, respectively. Show that the acceleration of gravity at the surface of Earth is 9.80 m/s2.

You know the mass of earth, and a well known principle (accept it) that for symetrical distributions, you can consider that mass at the center of a sphere.
g=G*Mass/r^2 
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