1. The problem is to find the sum of $-3$ and $-9$. 2. The formula for adding two numbers is simply $a + b$. 3. Here, $a = -3$ and $b = -9$. 4. Adding these, we get: $$-3 + (-9) = -3 - 9$$ 5. When adding two negative numbers, you add their absolute values and keep the negative sign: $$-3 - 9 = -(3 + 9) = -12$$ 6. Therefore, the final answer is $-12$.