1. **State the problem:** Solve the equation $678 - 9$. 2. **Formula and rules:** Subtraction is the operation of finding the difference between two numbers. Here, we subtract 9 from 678. 3. **Intermediate work:** $$678 - 9 = 669$$ 4. **Explanation:** We start with 678 and take away 9. This means we count backwards 9 steps from 678, which lands us at 669. 5. **Final answer:** $$\boxed{669}$$