find radius given gravitational force

Question

Mars has a mass of 6.4 x 10^23 kg and its moon Phobos has a mass of 9.6 x 10^15 kg. The magnitude of the gravitational force between the two bodies is 4.6 x 10^15N. Calculate the radius of the orbit between Mars and Phobos. 

Answer 1

You need to use the equation Fg=GmM/r². G is a constant 6.67x10^-11, the two m's are the mass of Mars and Phobos and Fg is the gravitational force provided. Then you solve for r which would be 9.44 x 10^6 m