A set of ordered pairs is a function if each domain value is only paired with one range value. That means each x-value can only match with one y-value. In other words, there is only one possible y-value that can go with any x-value.
Right now, those four sets of ordered pairs are a function because they all have different x-values, so there are no repeated x-values. However, if we added the point (2, 18) to the set, we'd have two points with the same x-value. (2, 11) and (2, 18) both have an x-value of 2, but one is paired with 11 and the other is paired with 18. In this case, there are two possible y-values that go with the x-value 2, so it is no longer a function.
The relation is not a function when the point (2, 18) is added to the set.
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