(c) Acceleration due to gravity at an altitude $h$ is
$g_{h} = \frac{gR_{e}^{2}}{\left( R_{e} + h \right)^{2}}$; where $R_{e}$ is the radius of the earth
$g_{h} = \frac{9.8m/s^{2} \times \left( 6400 \times 10^{3}m \right)^{2}}{\left( 6400 \times 10^{3}m + 520 \times 10^{3}m \right)^{2}} = 8.4m/s^{2}$