The expression you've provided is:

$(-9a^2b) - (+8a^2b)$

Let's simplify it:

1. Combine the like terms, which both contain $a^2b$: $-9a^2b - 8a^2b = (-9 - 8)a^2b = -17a^2b$

So, the simplified expression is:

$-17a^2b$

Would you like more details or explanations?

Here are some related questions to explore:

1. What are like terms in algebra, and how do you identify them?
2. How do you factor expressions with similar terms like $a^2b$?
3. What are common mistakes to avoid when simplifying algebraic expressions?
4. How would you simplify expressions involving different powers, such as $a^2$ and $a^3$?
5. How can you expand this process to work with more complex polynomial expressions?

Tip: Always ensure that terms are combined only if they are like terms, meaning they have the same variables raised to the same powers.