Which value of x will make this fraction undefined?

5/x-3

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).