You always invert the second number in a division question and then multiply. This one is a little different. It has three levels. What do you do about that?
Now you have a four level question which is handled the same way as all four level question.
Invert the bottom and multiply. Invert means turn upside down. So you turn the 2/1 upside down and you get 1/2
What you get is 1/18 The green box with the question mark is a 1.
Answer: Use a compass to measure the length of AB. Draw an arc from point B (or point A) with that distance. Extend line AB through that arc and label the intersection as point C. AC is twice the length of AB.