The result of $9 + (-9)$ is:

$9 + (-9) = 0$

Would you like further explanation on why adding a number and its negative results in zero?

Here are five related questions to deepen understanding:

1. What is the result of $12 + (-12)$?
2. How do you subtract a negative number from a positive number?
3. What is the additive inverse of a number?
4. If $a + b = 0$, what can be said about $a$ and $b$?
5. How does the concept of opposites work on a number line?

Tip: Adding a number and its negative (its additive inverse) always results in zero.