[tex]\bf \begin{array}{lllll}
&x_1&y_1&x_2&y_2\\
% (a,b)
&({{ 1}}\quad ,&{{ -2}})\quad
% (c,d)
&({{ 1}}\quad ,&{{ 3}})
\end{array}
\\\\\\
% slope = m
slope = {{ m}}= \cfrac{rise}{run} \implies
\cfrac{{{ y_2}}-{{ y_1}}}{{{ x_2}}-{{ x_1}}}\implies \cfrac{3-(-2)}{1-1}\implies \stackrel{und efined}{\cfrac{3+2}{0}}[/tex]
hmmm, the slope is undefined, that means, is a vertical line then, check the picture below.
