Answer
(a) $g = 9.72 ~m/s^2$
(b) $g = 2.43 ~m/s^2$
Work Step by Step
Let's use 6400 km as the radius of the earth.
(a) At 6400 m above the Earth's surface, $r = 6.4064 \times 10^6 ~m$
$g = \frac{GM}{r^2} = \frac{(6.67 \times 10^{-11} ~N \cdot m^2/kg^2)(5.98 \times 10^{24} ~kg)}{(6.4064 \times 10^6 ~m)^2}$
$g = 9.72 ~m/s^2$
(b) At 6400 km above the Earth's surface, $r = 1.28 \times 10^7 ~m$
$g = \frac{GM}{r^2} = \frac{(6.67 \times 10^{-11} ~N \cdot m^2/kg^2)(5.98 \times 10^{24} ~kg)}{(1.28 \times 10^7 ~m)^2}$
$g = 2.43 ~m/s^2$