Derive the relation between acceleration due to gravity (g) at the surface of a planet and gravitational constant “G”

Consider a body of mass “m” is placed on the surface of a planet of mass “M” and Radius
“R” From Newton’s gravitational Law 
 gravitational Force $\mathrm{F}=\frac{\mathrm{GMm}}{{\mathrm{R}}^2}$.... (1)
Weight of the body F = mg.... (2)
(1) &(2)$\cancel{\mathrm{m}}\mathrm{g}=\frac{\mathrm{GM}\cancel{\mathrm{m}}}{{\mathrm{R}}^2}$
$\mathrm{g}=\frac{\mathrm{GM}}{{\mathrm{R}}^2}$(G=universal gravitational constant)