[tex]\bf \begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&&(~{{ a}} &,&{{ c}}~)
% (c,d)
&&(~{{ m}} &,&{{ c}}~)
\end{array}
\\\\\\
% slope = m
slope = {{ m}}\implies
\cfrac{\stackrel{rise}{{{ y_2}}-{{ y_1}}}}{\stackrel{run}{{{ x_2}}-{{ x_1}}}}\implies \cfrac{c-c}{m-a}\implies \cfrac{0}{m-a}\implies 0[/tex]
