9-1/-2-(-1) = 9 + 1/2 + 1 = 10 + 1/2 = 21/2
You need to write this as:
or some such.
Also your slope formula is written incorrectly. It should be:
m = (y2 - y1)/(x2 - x1)
where your points are (x1, y1) and (x2, y2).
It may seem like I'm picking at trivial things, but I'm really not. After all, if you don't communicate clearly others won't be able to understand you.