Did you note that 2 will not work for a) and -3 will not work for b) because they make the denominators 0? If not, remember that division by 0 being undefined. You may have a little bit more difficulty with seeing that two will not work for c) if you factored the trinomial in the denominator.
Always keep in mind why the denominator can not be 0. Division by zero is undefined as there is no number that can be multiplied by 0 that results in a product other than zero.