# What is the slope of a line perpendicular to the y-axis?

The y-axis is a vertical line. A vertical line has a slope of $\frac{1}{0}$ which is $u n \mathrm{de} f$ or undefined.
The negative reciprocal would be $\frac{0}{1}$ or $0$.
So, the slope of the perpendicular would be $0$.
*note that the sign does not come into play because $0$ is neither positive nor negative.