to get the equation of any straight line, we simply need two points off of it, let's use the points in the picture below.
[tex](\stackrel{x_1}{-2}~,~\stackrel{y_1}{2})\qquad (\stackrel{x_2}{4}~,~\stackrel{y_2}{-1}) \\\\\\ \stackrel{slope}{m}\implies \cfrac{\stackrel{rise} {\stackrel{y_2}{-1}-\stackrel{y1}{2}}}{\underset{run} {\underset{x_2}{4}-\underset{x_1}{(-2)}}}\implies \cfrac{-3}{4+2}\implies \cfrac{-3}{6}\implies -\cfrac{1}{2}[/tex]
[tex]\begin{array}{|c|ll} \cline{1-1} \textit{point-slope form}\\ \cline{1-1} \\ y-y_1=m(x-x_1) \\\\ \cline{1-1} \end{array}\implies y-\stackrel{y_1}{2}=\stackrel{m}{-\cfrac{1}{2}}(x-\stackrel{x_1}{(-2)}) \\\\\\ y-2 = -\cfrac{1}{2}(x+2)\implies y-2=-\cfrac{1}{2}x-1\implies y=-\cfrac{1}{2}x+1[/tex]
