We have a vertical line x = 4.
If we define a function as y = f(x), then for f(x) to be a function, there is one and only value of f(x) for each value of x.
Taking that into account, in this relation, we have infinite values of f(x) for the same value of x.
For example, (4,0) and (4,10) are part of the relation. Both ordered pairs have the same value of x and different values of y, so they are enough proof to convlude that x = 4 is not a function.
