The slope is usually represented by rise/run, which means the amount of units you go up over the amount of units you go right. If you go down, it would be a negatove rise, and if you go left, it would be a negative run. When you have [tex] \frac{-}{+}[/tex] or a [tex] \frac{+}{-}[/tex] , that would make your slope negative, since your fraction is negative. If you have both negatives or both positives, it would be a positive slope because your answer is positive. In this case, we can take the points (-1,3) and (0,0) to find the slope. There are many ways to do it, but the easiest one for me if you already have a graph is to count how many units or numbers you go up and how many you go left and plug those in to the formula.
It'd be: for the rise (the amount of spaces from y to y) you start at +3 and end at 0, so the rise would be -3. It is negative be cause you are going down, not up like I said before. For the run (amount of spaces from x to x) you start at -1, and end at 0, so you move right 1 space. Your run is 1. This will give you a slope of [tex] \frac{-3}{1}[/tex] , which can, of course, be simplified to -3. Your slope is -3.
Good luck in math class!
