to get the slope of any line, we only need two points, in this case, we have them right there, so let's use that
[tex](\stackrel{x_1}{3}~,~\stackrel{y_1}{42.5})\qquad (\stackrel{x_2}{9}~,~\stackrel{y_2}{57.5}) \\\\\\ \stackrel{slope}{m}\implies \cfrac{\stackrel{rise} {\stackrel{y_2}{57.5}-\stackrel{y1}{42.5}}}{\underset{run} {\underset{x_2}{9}-\underset{x_1}{3}}}\implies \cfrac{15}{6}\implies \cfrac{5}{2}\impliedby \stackrel{\textit{what does it represent?}}{\begin{array}{llll} \textit{\$5 in the register}\\ \textit{for every 2 cups sold} \end{array}}[/tex]