C(9,3) = C(9,a) means choosing 3 items from a set of 9 is equal to choosing a number of items from the same set. Since the number of ways to choose 3 items from a set of 9 is 84, we have C(9,3) = 84. We can find the value of a by solving C(9,3) = C(9,a), which gives a value of a = 6. Therefore, x can be any number except 3 and 6.