A down & dirty way to do it: 15 & 19 can be divided by 3 (I know it is 18, just bear with me), so reduce 19 by 3, 15 by 3 and you get 16 & 12. But you want 15 for the long side, so 16-1=15, 12-1=11. Now you have 15 by 11, which isn't mathematically correct bu close enough for me.