The greatest common factor of two numbers is, as the name suggests, the greatest integer that divides both numbers.
You number are already factored in product of primes, so it is obvious that the first number 133 = 19x7 is divisible by 1, 7, 19 and 133.
The same goes for 15 = 3x5: it is divisible by 1, 3, 5 and 15.
As you can see, there is no non-trivial divisor which is common to both number, so the answer is 1
