Question what is the slope? What does the function represent?

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]