What is the slope of a line that is perpendicular to the line y = 1?

The line        Y = 1    is a horizontal line; its slope is zero.

A line perpendicular to   Y = 1   is a vertical line.
Its slope is undefined.
(This does NOT mean that it's infinite, or anything else.
The slope of a vertical line is undefined.  It doesn't have
a name or a symbol.  It's nothing.)

9×5 are 45

12 days