The domain of the function f(x) = 2x + 5 is all real numbers.
This is because the function is a linear function, and linear functions do not have any restrictions on their inputs. You can substitute any real number for x, and you will get a valid output. In mathematical terms, this means that there are no values of x that would make the function undefined.
To put it simply, if you take any number, positive or negative, you can plug it into f(x) and receive a corresponding output, which confirms that the domain is indeed all real numbers.