39 inches is the longest bc a foot is 12 inches and the longest feet the is 3 feet or a yard. So 12 x 3 = 36 and that is 3 inches short than 39 so 39 is the longest.
To check if it has a max or min, you must reference the a value. The a value in this case is 2, and that is positive. Positive parabolas point up, so they can only have a minimum point.