When you use the midpoint formula, (x1+x2/2 , y1+y2/2) you end up taking one away from -4, and dividing it by 2 (-3/2), and when you add 2 to -4, you get -2, divided by 2, is -1.
So in total, there are 7 marbles, (3 black + 4 red = 7 total marbles) and we want to find out how much black marbles are able to come out. So it would be 3/7. :) x
