physics question over gravity

Question

Mars has a mass of about 6.87 × 10^23 kg, and its moon Phobos has a mass of about 9.9 × 10^15 kg. If the magnitude of the gravitational force between the two bodies is 4.64 × 10^15 N, how far apart are Mars and Phobos? The value of the universal gravitational constant is 6.673 × 10^−11 N · m2 /kg2 . Answer in units of m.

Answer 1

F = GMm/r^2. From here r = (GMm/F)^1/2 = (6.673·10^-11N·m²/kg²·9.9·10¹⁵kg·6.87·10²³kg/(4.64·10¹⁵N))^1/2 =

9.89·10⁶ m