Solve the equation for $M$. \[ f=-\frac{G M n}{r^{2}} \]

Step 1 ：Given the equation \(f=-\frac{G M n}{r^{2}}\)

Step 2 ：We need to solve the equation for M

Step 3 ：Multiply both sides of the equation by \(-\frac{r^2}{Gn}\)

Step 4 ：The solution to the equation for M is \(\boxed{-\frac{f r^{2}}{G n}}\)