we can simply pick any two points off the x,y table, so
[tex]\bf \begin{array}{|cc|ll}
\cline{1-2}
x&y\\
\cline{1-2}
0&20\\
3&11\\
6&2\\
9&-7\\
\cline{1-2}
\end{array}
\begin{array}{llll}
\\[1em]
\leftarrow \textit{let's use this point}\\\\
\leftarrow \textit{and this point}
\end{array}~\hspace{5em}
(\stackrel{x_1}{3}~,~\stackrel{y_1}{11})\qquad
(\stackrel{x_2}{9}~,~\stackrel{y_2}{-7})
\\\\\\
slope = m\implies
\cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{-7-11}{9-3}\implies \cfrac{-18}{6}\implies -3[/tex]
