# What is the slope of a line that is perpendicular to a slope of undefined?

Its slope will be zero and it will be of the form $x = a$
Slope is undefined for a line, which is perpendicular to $x$-axis i.e. parallel to $y$-axis.
Hence, a line perpendicular to this line would be parallel to $x$-axis and
its slope will be zero and it will be of the form $x = a$.