Question 30251
Based on the definition of a function, there cannot be more than one f(x) value for the same x;

In example 1, Since x value is not duplicated (possible values of x are  1, 9, -1, and -9), the situation of more than one f(x) for the same x does not arise and hence it is a function.

In example 2, x value duplicates in {-2,4), {-2,7}).  For x = -2, the values of f(x) are 4 and 7;  So, this is not a function.