simply pick any two points off the table.
[tex]\bf \begin{array}{|c|cccc|ll} \cline{1-5} x&2&4&6&8\\ \cline{1-5} y&-10&20&50&80&\\ \cline{1-5} \end{array}~\hspace{5em} (\stackrel{x_1}{4}~,~\stackrel{y_1}{20})\qquad (\stackrel{x_2}{8}~,~\stackrel{y_2}{80}) \\\\\\ slope = m\implies \cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{80-20}{8-4}\implies \cfrac{60}{4}\implies 15[/tex]
