Chapter 1, Equations and Inequalities - 1.7 Solve Absolute Value Equations and Inequalities - 1.7 Exercises - Skill Practice - Page 55: 17

$$n=-19\quad \mathrm{or}\quad \:n=1$$

Since the value inside the absolute value can be positive or negative and get the same number, we must consider both options. Doing this, we find: $$n+9=-10\quad \mathrm{or}\quad \:n+9=10\\ n=-19\quad \mathrm{or}\quad \:n=1$$