That depends if the line is vertical or horizontal. If the line is vertical, the slope (steepness) of the line is undefined (this is because the x values across the entire line are the same). If the line is horizontal, the slope of the line is 0 (this is because the y values will always subtract to equal 0 when you use the form y1 - y2/x1 - x2 to find slope).
Hope this helps.