Final answer:
The domain of a function represented by a horizontal line graphed from x = -1 to x = 5 would be all real numbers from -1 to 5, inclusive, which can be denoted as (-1, 5).
Step-by-step explanation:
When determining the domain of a function, we are looking for all possible values of x for which the function is defined. In the case of a function represented by a horizontal line, the function is defined for all real numbers between the two endpoints, inclusive, if the endpoints are specified. If the question is referring to the domain of a horizontal line graphed between x = -1 to x = 5, the domain would be all real numbers between -1 and 5, including -1 and 5.
Therefore, the domain of the function is (-1, 5). Note that this does not include the entirety of the real number line, as values less than -1 or greater than 5 are not part of the domain as specified by the conditions of the function.