1. Stating the problem: Calculate $(-9) + (-9)$. 2. Explanation: Adding two negative numbers means combining their absolute values and keeping the negative sign. 3. Calculation: $|-9| = 9$ and $|-9| = 9$. 4. Add the absolute values: $9 + 9 = 18$. 5. Attach the negative sign back since both numbers were negative: $-18$. 6. Final answer: $$(-9) + (-9) = -18$$.