Here's another way to do this....it's a "trick" I used to use when I tutored...
Take the points and "reverse" them.....we have (2, -1) and (3, 3)
Line them up like this and draw a line under them
(2 , -1)
(3 , 3)
______ Now just "subtract" the "underneath" things from the things "overhead" - individually - like this
(2 , -1)
(-3 , - 3)
_______
-1 , -4
Now.......put the thing in the first column "over" the thing in the second column (-1) / (-4) = 1 / 4 and Voila!!.......there's the "slope"
Let's take a look at this.