The first step in simplifying this expression is to identify which, if any, terms can be combined. Remember, only like terms — constant terms or terms with the same variable and the same exponent — can be combined.
10ab + 2ab^2 - 9ab
In this case, we have two ab-terms and one ab^2-term. Only the ab-terms can be combined, so to simplify the expression we will rearrange it according to the Commutative Property of Addition and then combine like terms.