The expression to evaluate is $x + 9y$ with the values $x = -3$ and $y = -4$.

Let's substitute the values into the expression:

$x + 9y = (-3) + 9(-4)$

Now, calculate:

$x + 9y = -3 + (-36) = -3 - 36 = -39$

Thus, the expression evaluated at the given values is $\boxed{-39}$.

Would you like more details or further clarification?

Here are 5 related questions:

1. How would the result change if $y = -3$ instead of $y = -4$?
2. Can you explain how the distributive property applies to this expression?
3. What would the expression $x + 9y$ look like if you generalized it for any values of $x$ and $y$?
4. How would the result differ if the expression was $9x + y$ instead of $x + 9y$?
5. What would happen if you added a third variable, such as $z$, into the expression?

Tip: Always perform operations step by step to avoid mistakes, especially when dealing with negatives and parentheses.