Solution:

Gravitational acceleration = g = 4.0 $ms^{-2}$
        Radius of Earth = $R_{e}$ = 10000 km = $10000 \times 1000$ m = $10^{7}$m
        Mass of Earth = $M_{e}$ =?

        $M_{e}$ =$\frac{(gR^{2})}{G}$ 
        $M_{e}$ = $\frac{(4.0 \times (10^{7})^{2})}{(6.673 \times 10^{-11})}$ 

              =$\frac{(4.0\times 10^{14})}{(6.673 \times 10^{-11})}$ 
              = $0.599\times10^{25}$ kg = $5.99\times 10^{24}$ kg