Explanation:
there is really only one rule to make a relationship between 2 sets of values a function :
every defined value of x must have exactly one assigned value of y. not 0, not 2 or more. exactly one.
therefore, this here is not a function, because x = 2 has 2 different assigned result (y) values.
in essence, if x = 2, we don't know what the function value would be. that's why.