1. **Problem statement:** Solve the equation $x + 9 = 12$ for $x$. 2. **Step 1: Subtract 9 from both sides to isolate $x$ on the left side.** $$x + 9 - 9 = 12 - 9$$ 3. **Simplify both sides:** $$x + \cancel{9} - \cancel{9} = 12 - 9$$ $$x = 3$$ 4. **Final answer:** $$x = 3$$ This is the solution to the equation by using the equivalence transformation of subtracting the same number from both sides.