[tex]\bf \begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&&(~ 4 &,& 1~)
% (c,d)
&&(~ 2 &,& 8~)
\end{array}
\\\\\\
% slope = m
slope = m\implies
\cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{8-1}{2-4}\implies \cfrac{7}{-2}\implies -\cfrac{7}{2}[/tex]
