x = 12 is a vertical line. a parallel line will also have to be a vertical line, and a perpendicular one will, in this case, be a horizontal line.
the slope of x = 12 is undefined, because it fails the vertical line test. a horizontal line, however, passes the vertical line test -- but there is no increase or decrease. the slope of a horizontal line is 0.
so with that information in mind (the slope of a vertical line is undefined, the slope of a horizontal line is 0), you can easily state that a line parallel to x = 12 will have an undefined slope, and also that a line perpendicular to x = 12 will have a slope of 0.
