A line is y=12. what is the slope of a line perpendicular to this

Writing this equations in slope intercept form we get y=0x+12, from which we see the slope I'd 0. the slope of perpendicular lines are negative reciprocals. the negative receipt of 0 would be - 1/0, but we know that division by 0 is undefined. there for the slope of the lines are 0 and undefined. sometimes it is useful to say the slope of a vertical line is infinity.