If the 38 inches is the diagonal of your square (which is how I read it) the sides of the square need to be 26 7/8 inch.
The formula is A squared plus B squared = the square root of the diagonal. Or, as this is a square, 2 times A times A (because A = B) = the square root of the diagonal. So if you take your 38 inches and square it (multiply 38 times 38) then divide that by 2 you have the number that if you find what multiplies itself to get that number that is the length of the side. Called the square root. Do not try this without a calculator or spread sheet. Confused? I don't blame you. It's harder to describe than to do. The number is 26 7/8 in this case.
tim in san jose