The line y = 5 is a horizontal line (with a slope of 0)
Perpendicular to horizontal is vertical
Vertical lines represent all the points with a certain x coordinate. The equations for these lines have the form:
x = number
where "number" is the x coordinate all the points on the line share.
If the point (6, -10) is to be on this vertical line, then its x coordinate, 6, is the x coordinate all the points on the lines share