Final answer:
The value of x that makes the given expression undefined is x = 4.
Step-by-step explanation:
The given expression x+4/x-4 is undefined when the denominator is equal to zero. In other words, when x-4 = 0. To find the value of x, we can solve this equation:
x-4 = 0
Adding 4 to both sides of the equation, we get:
x = 4
Therefore, the value of x that makes the given expression undefined is x = 4.