Well, let's see. A prime number is a number that can only be divided evenly by itself or one (and cannot be divided evenly by 2, 3, 4, 5, etc.).13x3=39 (13 and 3 were both prime) but this is not high enough. 17x3=51 (still not hish enough). 19x3=57.
That's the answer: 19 and 3.