Let's solve the equation:

$x + 15 = 9$

To isolate $x$, subtract 15 from both sides:

$x = 9 - 15$

$x = -6$

So, the solution is $x = -6$.

Would you like a more detailed explanation or have any questions about this? Here are a few related questions you might find useful:

1. How do we solve equations with more complex terms on both sides?
2. What happens when we have negative numbers in an equation?
3. How do we check if our solution is correct?
4. What are the properties of equality that help solve equations?
5. How do we solve equations involving fractions or decimals?

Tip: Always aim to isolate the variable (like $x$) by performing the same operation on both sides of the equation!