First, you would determine that both point A and point B would have the same y-coordinate of 1. Then you would determine the difference between the x values of points A and B. Since 5 (the x-value for B) minus 2 (the x-value for A) is 3, the legnth of side AB would be 3 units.
It's actually 6 cm. This is because the diameter of the smaller circle is 12pi/pi which is 12, and the diameter of the bigger one is 18pi/pi which is 18. 12 + 6 = 18, so the diameter increases by 6.