In a function, each input (x-value) must correspond to exactly one output (y-value). This means that no two ordered pairs can have the same x-value.
Looking at the given relation:
{(2, 8), (4, 5), (7, 3), (9, 0), (?, 6)}
The x-values currently present in the pairs are 2, 4, 7, and 9. If we add another ordered pair with the missing x-value being 7, it would mean that the x-value 7 would correspond to two different y-values (3 and 6), which would violate the definition of a function.
Therefore, the value that is not allowed for the missing x-value is:
7