### Gravitational force

Mars has a mass of about 6.4 x 10^23 kg, and its moon Phobos has a mass of about 9.6 x 10^15 kg. If the magnitude of the gravitational force between the two bodies is 4.6 x 10^15 N, how far apart are Mars and Phobos? Please show the work, answer and the formulas you used. Thanks!