The line y = 12 has a slope of 0 because it is a horizontal line, meaning that for any change in x, there is no change in y. Ths means the slope of a line parallel to it would be 0 (since parallel lines have the same slope), and the slope of a line perpendicular to it would be undefined (since perpendicular lines have slopes that are negative reciprocals of each other, and when you divide by 0, you get an undefined result).
