AB=9 centimeters, you figure out what times what is 9. in this case, it is 3 for A and B. Then you go to BC=12 and 3 for B and 4 for C making it 12 So AC would equal 3x4.
Assuming this is on a line segment AC would be the full length of the line segment that you are trying to find and point B would be splitting in between. So if the distance between AB is 9 and the distance between BC is 12 then you just add them up the get the full length of AC which is 21
