[tex]\bf \begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&&(~ -5 &,& 2~)
% (c,d)
&&(~ -5 &,& 6~)
\end{array}
\\\\\\
% slope = m
slope = m\implies
\cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{6-2}{-5-(-5)}\implies \cfrac{6-2}{-5+5}\implies \stackrel{und efined}{\cfrac{4}{0}}[/tex]
now, an undefined slope, simply means a vertical line, check the picture below.
