slope

You can find the slope of a horizontal line like finding any other slope of a line, but the outcome is always zero if you do it correctly :)

M = (y1 - y2)/(x1 - x2)

it's zero

Thanks, guest.....the reason for  Guest's answer is that......

slope =   [ change in y's ] / [ change in x's ]   

But....on a horizontal line, the y values are constant, so there is no change in them......only the x values change......

Another more intuitive explanation is that a horizontal line neither "rises" nor "falls."......Thus, it's "slope"  =  0

Because the equation for slope is Δy/Δx and since it is flat there is no change in y.  So that means it is 0.