The equation representing a line perpendicular to y=0 is x=a, where a is any constant value. This represents a vertical line with an undefined slope.
The equation that represents a line which is perpendicular to y = 0 is x = a, where a is any real number. The horizontal line y = 0 is the x-axis itself, and has a slope of 0.
A perpendicular line to this must be a vertical line, which cannot be described by the form y = mx + b (the slope-intercept form of a line) because it does not have a defined slope.
Instead, a vertical line's equation is given by x = a, where a is the x-intercept of the line. A vertical line has an undefined slope because it rises infinitely while not running horizontally at all.
The probable question may be:
What equation represents a line which is perpendicular to y=0?