Question

derive the formula of g

Answer 1

Let M bet the mass of the Earth and R be it's radius. Consider a body of mass 'm' lying on the surface of the Earth. Then, from Newton's law of gravitation,
              F = (GMm)/R²
If g is acceleration produced by this force, then. 
           g = F/m
      or, g = (GMm)/(R²m)
      or, g = (GM)/R²

Answer 2

This is your answer and g is Gravity ok
Hope it help u