well, let's take 2 points from it and get it :) , say let's use (4 , 5) and (-1 , 5)
[tex](\stackrel{x_1}{4}~,~\stackrel{y_1}{5})\qquad (\stackrel{x_2}{-1}~,~\stackrel{y_2}{5}) ~\hfill \stackrel{slope}{m}\implies \cfrac{\stackrel{rise} {\stackrel{y_2}{5}-\stackrel{y1}{5}}}{\underset{run} {\underset{x_2}{-1}-\underset{x_1}{(4)}}}\implies \cfrac{0}{-5}\implies 0[/tex]
bearing in mind that for any horizonal line it's always 0.
