When you are adding a positive and a negative it will always equal a positive. It depends because if the negative number is greater than the positive number than its a negative if u are adding them together. But if the positive number is greater than the negative number and you are adding them together than it will equal a positive. So, or you just switch the addition symbol to a subtraction symbol.
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.