The equation to find the slope (m) is:
y1-y2
--------
x1-x2
What that means is the change in y over the change in x.
After plugging in the numbers, the equation looks like this-
3-5
-----
4-8
3-5= -2
-----------
4-8= -4
So, the slope is 2/4 because the two negatives cancel each other out.
2/4 can be simplified to 1/2.
Slope = 1/2