Taking the square root of a negative number is not possible unless you involve complex numbers. If your teacher has not introduced that topic yet, then you just simply say "cannot be done" or something along those lines.
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.
