Chapter 6 - Section 6.6 - Rational Equations and Problem Solving - Exercise Set - Page 387: 18

$M=\dfrac{Fr^2}{-Gm}$

Using the properties of equality, in terms of $ M ,$ the given equation, $ F=\dfrac{-GMm}{r^2} ,$ is equivalent to \begin{array}{l} Fr^2=-GMm \\\\ \dfrac{Fr^2}{-Gm}=M \\\\ M=\dfrac{Fr^2}{-Gm} .\end{array}