Final answer:
The values of a that make the rational expression undefined are 9 and -9.
Step-by-step explanation:
The question asks for the values of a that make the rational expression undefined. To determine this, we need to identify any values of a that would result in a denominator of 0 in the rational expression. In this case, the values of a that make the expression undefined are 9 and -9, as these values would make the denominator equal to 0.