First, you find the length and width which are 16 and 12. Using the Pythagorean theorem (a^{2}+b^{2}=c^{2}) , you can figure it out. We know A and B, 16^{2}+12^{2}=c^{2}. Go on further and you get 256+144=c^{2} then 400=c^{2} and finally c=20. The diagonal is 20 units.