To simplify the expression $-4g^2 - 5g^2$, you can combine the like terms:

$-4g^2 - 5g^2 = (-4 - 5)g^2$

$= -9g^2$

So, the simplified expression is:

$-9g^2$

Would you like more details or have any questions?

Here are 5 related questions you might find useful:

1. How do you combine like terms in algebraic expressions?
2. What are like terms, and why can they be combined?
3. How do you simplify expressions with multiple variables?
4. What is the difference between combining like terms and factoring?
5. How can simplification help in solving algebraic equations?

Tip: Always look for like terms when simplifying expressions, as combining them can make complex problems much easier to handle!