check the picture below.
since the y-axis is a North-South line, the perpendicular to that will be an East-West line, as you see in the picture.
hmmm what's its slope anyway, well, let's pick two points to get it, hmmm we have -4, -2, and let's see from the graph -1,-2 will do.
[tex]\bf (\stackrel{x_1}{-4}~,~\stackrel{y_1}{-2})\qquad
(\stackrel{x_2}{-1}~,~\stackrel{y_2}{-2})
\\\\\\
% slope = m
slope = m\implies
\cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{-2-(-2)}{-1-(-4)}
\\\\\\
\cfrac{-2+2}{-1+4}\implies \cfrac{0}{3}\implies 0[/tex]
