The upper right-hand corner of a piece of paper, 9 in. by 4 in., as in the figure, is folded over to the bottom edge. How would you fold it so as to

The upper right-hand corner of a piece of paper, 9 in. by 4 in., as in the figure, is folded over to the bottom edge. How would you fold it so as to minimize the length of the fold? In other words, how would you choose x to minimize y?