any equation with only an x or y value present will either be a vertical or horizontal line. Because the y axis is vertical, a line that is present on y=-12 will sit on y=-12 forever, and therefore be a horizontal line
If any quantity is set equivalent to <em>y</em>, then it is a horisontal line. In other wourds, it is considered a zero <em>rate of change</em> [<em>slope</em>].