to get the the equation of any straight line, we only need two points off of it, let's use the two points already in the picture.
[tex](\stackrel{x_1}{2}~,~\stackrel{y_1}{10})\qquad (\stackrel{x_2}{0}~,~\stackrel{y_2}{0}) ~\hfill \stackrel{slope}{m}\implies \cfrac{\stackrel{rise} {\stackrel{y_2}{0}-\stackrel{y1}{10}}}{\underset{run} {\underset{x_2}{0}-\underset{x_1}{2}}}\implies \cfrac{-10}{-2}\implies 5 \\\\\\ \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}{10}=\stackrel{m}{5}(x-\stackrel{x_1}{2}) \\\\\\ y-10=5x-10\implies y=5x[/tex]
