When subtracting two numbers we need to subtract their absolute values and maintain the signal of the one that is farther from 0, which has the highest absolute value. On this case we can check the distance from each point to 0. The point B is 5 ticks away from 0, while the point A is 8 ticks away from 0, so the sign to be used is the one from point A, which is negative. The sign of "B-A" is negative.