1. **Stating the problem:** Solve the equation $x + 9 = 12$ for $x$. 2. **Formula and rules:** To isolate $x$, we use the rule that whatever we do to one side of the equation, we must do to the other side to keep it balanced. 3. **Step-by-step solution:** Start with the equation: $$x + 9 = 12$$ Subtract 9 from both sides to isolate $x$: $$x + 9 - 9 = 12 - 9$$ Simplify both sides: $$x + \cancel{9} - \cancel{9} = 12 - 9$$ $$x = 3$$ 4. **Explanation:** We subtracted 9 from both sides because $x$ is being added to 9. Subtracting 9 cancels out the +9 on the left side, leaving $x$ alone. On the right side, we subtract 9 from 12 to keep the equation balanced. **Final answer:** $$x = 3$$