If you haven't got, or don't know, the slope of a line, what else can define a line? Knowing one point isn't enough because many lines will pass through that point. Knowing two points is enough, because you can join them to give you a line. That done, you can find the slope, by taking the difference between the rectangular coordinates. Divide the difference between the vertical (usually called y) by the difference between the horizontal (usually called x) and you have the slope. The slope is positive if it looks like / and negative if it looks like \.