Slope of a line
Slope is rise over run: how much y changes for each step in x.
A worked example
Find the slope through (1, 2) and (4, 11). Rise = 11 - 2 = 9. Run = 4 - 1 = 3. Slope = 9/3 = 3.
The mistake people actually make
Keep the points in the same order top and bottom. Swapping one but not the other flips the sign.