My dear school students... we help you to get understanding about derivations in physics and chemistry..

Mass of Earth :

From the expression  g=GM/R^2
         M=(gR^2)/G
         M=( 9.8 x (6.38 x 10^6)^2 ) / 6.67 x 10^-11
         M=5.98 x 10 ^24 kg