[tex]\bf \begin{array}{lllll}
&x_1&y_1&x_2&y_2\\
% (a,b)
&({{ -8}}\quad ,&{{ 5}})\quad
% (c,d)
&({{ -8}}\quad ,&{{ -5}})
\end{array}
\\\\\\
% slope = m
slope = {{ m}}= \cfrac{rise}{run} \implies
\cfrac{{{ y_2}}-{{ y_1}}}{{{ x_2}}-{{ x_1}}}\implies \cfrac{-5-5}{-8-(-8)}
\\\\\\
\cfrac{-10}{-8+8}\implies \stackrel{und efined}{\cfrac{-10}{0}}[/tex]
that simply means, is a vertical line, check the picture below.
