Look at two perfect points like (-2,-2) and (0,1) Then use rise/run triangle to find the slope in the quickest way. Start from point (-2,-2), move 3 units up and 2 units to the right to reach point (0,1).
You do 12*3 because if 1 sheet of poster board can make 12 post cards, and you have 3 poster boards you will need 3 lots of 12 which is equivalent to 3*12!