Graphing Horizontal and Vertical Lines

One way we can determine if the given function is linear, is to graph it. The function $y=5$ is comprised of infinitely many points. In all of these points, $x$ can be any value, and $y$ must always be $5.$ For example, $(\text{-}4,5),(0,5),$ and $(2,5)$ will all be points of the function.

From the graph we can see that $y=5$ is a horizontal line. All horizontal lines have a slope of $0.$ As such, it is a linear function.