They're asking you to what value the quotient tends when x will be about to equal a. It's called the limit of the expression when x tends to a.
For this thing, you can factorize (a²-x²) with the (a-b)(a+b)=a²-b² formula. And as , , you can simplify ;-)
If you don't understand the notion of limit, it's like replacing x by a.