In these ordered pairs the first element (number) is an argument, and the second is the corresponding value. For this relation to be a function (which means one-valued function), each argument must have only one corresponding value.
This given relation has four arguments: `2,` `-9,` `4` and `5,` and they are all different. So now it is a (one-valued) function. To make this relation not a one-valued function, we have to add a pair which has an argument coinciding with one of these four and a different value for this argument.
The only suitable pair is (2, 18). It gives a different value (18 instead of 11) for the same argument (2).