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
Just because 2 things are related doesn't mean they will both be changed when one thing happens. For example steak and candy, just because they are both food doesn't mean they taste the same or interact with each other.
