The answer is 10.5 inches for width
First you can add 8.5 + 8.5 = 17 because the opposite side of the 8.5 is also 8.5.
After You have to divide subtract 17 from 38 which equals 21. Then you divide by 2 which equals 10.5.
For perimeter you need to add all the sides so we add 8.5 + 8.5 + 10.5 + 10.5.