to get the slope of any straight line we only need two points off of it, let's use the ones in the picture below.
[tex](\stackrel{x_1}{-1}~,~\stackrel{y_1}{2})\qquad (\stackrel{x_2}{4}~,~\stackrel{y_2}{-8}) \\\\\\ \stackrel{slope}{m}\implies \cfrac{\stackrel{rise} {\stackrel{y_2}{-8}-\stackrel{y1}{2}}}{\underset{run} {\underset{x_2}{4}-\underset{x_1}{(-1)}}}\implies \cfrac{-10}{4+1}\implies \cfrac{-10}{5}\implies -2[/tex]
