In a function, every x-value must be associated with only one y-value.
In the first table, we can see that the x-value x = 1 has two different y-values associated, y = 3 and y = -3. Then, this table doesn't represent a function.
Similarly, in the second table, x = 2 and x = -2 each have two y-values associated, and in the fourth table, x = 4 and x = 4 each have two y-values associated. Therefore, they are not functions.
On the other hand, in the third table, there is only one y-value associated with each x-value. In consequence, it is a function.
