When x=(3). Assuming f(x)=5/(x-3) and y=f(x), then the fraction is undefined when x=3, as (3-3)= 0, so.. 5/0. This is due to the fact that any number divided by a Zero is undefined ( # / 0) because it means essentially-- Number/Zero = (An unkown number multiplied by the denominator to give that specified numerator in the equation. And there is no known number-- (#)*(0) --(In this case) that will give the number (5).
