The expression you've provided in the image is:

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

We can simplify this step-by-step:

1. First, distribute the negative sign over the second term: $-(-8a^2b) = +8a^2b$ Now the expression becomes: $-9a^2b + 8a^2b$

2. Combine the like terms: $(-9 + 8)a^2b = -1a^2b$

Thus, the simplified expression is: $-a^2b$

Would you like further details or have any other questions?

Here are some related questions to deepen understanding:

1. How do you combine terms with different coefficients but the same variables?
2. What happens if you had powers other than 2 in the expression?
3. How would the expression change if the variables had different exponents?
4. Can you factor a common term out of a similar expression?
5. How do we approach simplification when there are multiple variables?

Tip: Always distribute any negative signs or coefficients carefully before combining like terms.