For what real values of a is the expression (a+3)/(a^2-4) undefined? List your answers in increasing order separated by commas.
We can factor the denominator first: (a+2)(a-2), and leave the numerator as it is: a+3. Now, think about what undefined means, and see what value of x can fit in.
More of a hint: you cannot divide by 0, that is impossible. So if the bottom is 0 the answer would be undefined.