[tex]\bf \begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&&(~{{ 9}} &,&{{ -7}}~)
% (c,d)
&&(~{{ 9}} &,&{{ -10}}~)
\end{array}
\\\\\\
% slope = m
slope \implies
\cfrac{\stackrel{rise}{{{ y_2}}-{{ y_1}}}}{\stackrel{run}{{{ x_2}}-{{ x_1}}}}\implies \cfrac{-10-(-7)}{9-9}\implies \cfrac{-10+7}{9-9}\implies \stackrel{und efined}{\cfrac{-3}{0}}[/tex]
with an undefined slope, that means is a vertical line, check the picture below.
