This is undefined when x=4 because the denominator would then be 0.

Factor x out of the numerator:



Factor what is in the parentheses:



This is undefined when x = 4

Now as long as x is not 4, we may cancel the 's, so

the function can be written as

, 

The graph of the function is below and the graph does not contain the
point (4,28).  So there is a point discontinuity at x=4.  Sometimes
this is called a "removable" discontinuity.



Edwin