Chapter 1, Equations and Inequalities - 1.7 Solve Absolute Value Equations and Inequalities - 1.7 Exercises - Skill Practice - Page 55: 28

$$k=-5\quad \mathrm{or}\quad \:k=9$$

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: $$6-3k=-21\quad \mathrm{or}\quad \:6-3k=21 \\ k=-5\quad \mathrm{or}\quad \:k=9$$