Check the picture below.
so, say let's use two points on that line to see what's its slope then.
[tex]\bf (\stackrel{x_1}{-6}~,~\stackrel{y_1}{-8})\qquad (\stackrel{x_2}{4}~,~\stackrel{y_2}{-8}) \\\\\\ slope = m\implies \cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{-8-(-8)}{4-(-6)}\implies \cfrac{-8+8 }{4+6}\implies \cfrac{0}{10}\implies 0[/tex]
