Hi my name is Vanessa. I think I may know the answer to your question. Please know that I started to learn this about a few weeks ago, and I am still learning. I think the answer is 6 units.
Show Your Work: AB equals 18 units and AD equals 12 units. 18 units - 12 units = 6 units.
*Please let me know if I got the answer right or wrong!! Thank you!!!!