What is the slope of the line that is parallel to the y-axis and passes through the point (–1, 5)?
check the picture below.
the y-axis is a vertical line, thus any line parallel to it, will also be a vertical line, so, let's pick another point, hmmmmm say -1, 3, thus
[tex]\bf \begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&&(~ -1 &,& 5~)
% (c,d)
&&(~ -1 &,& 3~)
\end{array}
\\\\\\
% slope = m
slope = m\implies
\cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{3-5}{-1-(-1)}\implies \cfrac{3-5}{-1+1}\implies \stackrel{und efined}{\cfrac{-2}{0}}[/tex]
