In a graph what function is a nonvertical line or part of a nonvertical line.

A function cannot have multiple outputs (y-values) for any given input (x-values). For example, y=2x+3 is a function because no mater what number x represents, there is a single, unique y-value (When x=3 y=9). In other words, you can never have two (or more) y-values that correspond to a single x-value. Therefore, a vertical line, for example x=2, cannot be a function because it has an infinite amount of y-values for a single x-value. A horizontal line is a function because there is only one y-value for any given x-value. For example, y=3. No mater what x-value is used, there is only one y-value possible (in this case, y will always equal 3). I am, of course, referring to the cartesian coordinate system, also known as the rectangular coordinate system