In order to have a function, each value of x in the set of points can have only one corresponding value of y.
Looking at the set of points, the value of x = 2 has two different values of y: 0 and 2.
Therefore this relation is not a function.
Graphing these points, we can see that the point (2, 2) is above the point (2, 0), that is, they are in the same value of x, showing that one value of x has two values of y, therefore we don't have a function in this case.