An undefined slope belongs to a perfectly vertical line. Because this is the case, there are no changing x values for each value of y. When x = 5, y = -3; when x = 5, y could equal - 2; when x = 5, y could equal -1; etc. The equation for this line only has to include the x coordinate. Our line is x = 5. IF this was a line with a slope of 0, it would be a perfectly horizontal line and only the y coordinate would be considered. That line would be y = -3. See the difference?
