(a) g = Gm/r^2
where G=6.67 * 10^-11 Nm^2/kg^2,
m is the mass of the planet,
r is the radius of the planet.
7.0 = (6.67 * 10^-11)/(2500^2) * m
m= (25000000^2 * 9.0)/(6.67 * 10^-11)
m = 8.4333 x 10^24 kg
(b) g=Gm/r^2. This time, the radius is 2500+10000 = 12500km g = (6.67 * 10^-11 * 8.43333*10^18)/(12500^2)
g = 3.6 m/s^2