[tex]\bf \begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&&(~{{ -1}} &,&{{4}}~)
% (c,d)
&&(~{{ -1}} &,&{{ -4}}~)
\end{array}
\\\\\\
% slope = m
slope \implies
\cfrac{\stackrel{rise}{{{ y_2}}-{{ y_1}}}}{\stackrel{run}{{{ x_2}}-{{ x_1}}}}\implies \cfrac{-4-4}{-1-(-1)}\implies \cfrac{-4-4}{-1+1}\implies \stackrel{und efined}{\cfrac{-8}{0}}[/tex]
check the picture below.
