2 is a value of x that would not make the set a function because a function can't have multiple coordinates wit hthe same domain (x-coordinate). 2 is a domain that has already been used in the point (2,7). If this is used in (x,5), the point would become (2,5) and repeat a domain.
Note:
The x-coordinate could also be 1 or 6 because they have been used as domains as well.
