Answer:
x = 5
Explanation:
You want to know what number must be excluded from the domain of f(x) = 15/(x -5).
Domain
The domain of a function is the set of numbers for which it is defined. The function f(x) = 15/(x-5) will be undefined when the denominator is zero:
x -5 = 0
That will be the case when x = 5.
The domain of f must exclude x=5.