The slope of a horizontal line is 0.

A quick way to see why: slope is defined as “rise over run,” or slope=Δy/Δx\text{slope}=\Delta y/\Delta xslope=Δy/Δx. For a horizontal line, the yyy-value does not change as you move left or right, so Δy=0\Delta y=0Δy=0. That makes the fraction 0/Δx=00/\Delta x=00/Δx=0, so the line’s slope is zero.