What is the slope of a line that is perpendicular to a slope of 0?

A line that has a of ##0## has no rise and is therefore a horizontal line. The perpendicular line to this would be a vertical line whose slope is undefined as there would be ##0## run.

A slope of ##0## is a horizontal line with no rise. This line is represented as ##y =##

An undefined slope is a vertical line with no run. This line is represented as ##x=##