What is the lowest output value approached by the function f (x) = 2x-5?

Are you sure you wrote it right? : - (

The function f (x) = 2x-5 is a linear function.

Unless otherwise stated (and that does not seem to be the case) in the real numbers domain, x may assume any value.

The same can be said to y.

So, the lowest output value approached by this function is - ∞

(minus infinity).

:-)