[tex]\bf \begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&&(~ \frac{2}{3} &,& \frac{4}{7}~)
% (c,d)
&&(~ \frac{2}{3} &,& \frac{11}{7}~)
\end{array}
\\\\\\
% slope = m
slope \implies
\cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{\frac{11}{7}-\frac{4}{7}}{\frac{2}{3}-\frac{2}{3}}\implies \cfrac{\frac{7}{7}}{0}\implies \cfrac{1}{0}\impliedby un de fined[/tex]
