1. The problem is to simplify the expression $-9-9$. 2. This is a subtraction problem involving two integers. 3. When subtracting a positive number, you move to the left on the number line. 4. Calculate $-9-9$ by adding the negative values: $$-9-9 = -9 + (-9) = -18$$ 5. So, the final answer is $-18$.