Answer:
sqrt68 or around 8.25
Explanation:
Use the inverse of the Pythagorean theorem being c^2-b^2 = a^2
18^2 - 16^2 = AD or 324-256 = AD which gives AD = sqrt 68 which is around 8.25. Since it is a cuboid, the side parallel to it is equal in length so the answer is the sqrt of 68 or around 8.25.