1. **State the problem:** Solve the equation $$-ax + 6 = -21$$ for $$x$$. 2. **Isolate the term with $$x$$:** To isolate $$x$$, subtract 6 from both sides of the equation using the subtraction property of equality. $$-ax + 6 - 6 = -21 - 6$$ $$-ax = -27$$ 3. **Solve for $$x$$:** Next, divide both sides of the equation by $$-a$$ using the division property of equality. $$\frac{-ax}{-a} = \frac{-27}{-a}$$ $$\cancel{-}x = \frac{27}{a}$$ So, $$x = \frac{27}{a}$$ 4. **Find $$x$$ when $$a = 3$$:** Substitute $$a = 3$$ into the expression. $$x = \frac{27}{3} = 9$$ **Final answer:** $$x = \frac{27}{a}$$ and for $$a=3$$, $$x=9$$.