same thing here, let's pick two points, any two points off the table to get the slope.
[tex]\bf \begin{array}{|cc|ll} \cline{1-2} tables&cost\\ \cline{1-2} 1&10\\ 2&18\\ 3&24\\ 4&28\\ \cline{1-2} \end{array} \begin{array}{llll} \\[1em] \leftarrow \\\\ \leftarrow \end{array}\qquad \qquad (\stackrel{x_1}{2}~,~\stackrel{y_1}{18})\qquad (\stackrel{x_2}{4}~,~\stackrel{y_2}{28})[/tex]
[tex]\bf \stackrel{\textit{\large average rate of change}}{\stackrel{slope}{m}\implies \cfrac{\stackrel{rise} {\stackrel{y_2}{28}-\stackrel{y1}{18}}}{\underset{run} {\underset{x_2}{4}-\underset{x_1}{2}}}\implies \cfrac{10}{2}\implies 5}[/tex]
