check the picture below.
so, let's pick another point on that line, say -3, 0, just to check its slope.
[tex]\bf \begin{array}{lllll}
&x_1&y_1&x_2&y_2\\
% (a,b)
&({{ -3}}\quad ,&{{ 3}})\quad
% (c,d)
&({{ -3}}\quad ,&{{ 0}})
\end{array}
\\\\\\
% slope = m
slope = {{ m}}= \cfrac{rise}{run} \implies
\cfrac{{{ y_2}}-{{ y_1}}}{{{ x_2}}-{{ x_1}}}\implies \cfrac{0-3}{-3-(-3)}
\\\\\\
\cfrac{0-3}{-3+3}\implies \stackrel{und efined}{\cfrac{-3}{0}}\impliedby \textit{thus a vertical line at -3}[/tex]
