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.
![\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](https://img.qammunity.org/2019/formulas/mathematics/middle-school/muipl0p7o7ia9pqxf03n8bpkpybewd8umc.png)