The greatest common factor is 22.
22 x 17 = 374 and 22 x 29 = 638.
One way to find this to to use the Euclidean algorithm (named in honor of Euclid).
Divide the smaller number into the larger number, and look at the remainder:
638 ÷ 374 = 1, with a remainder of 264 (Actually, you don't care about the quotient, just the remainder).
Since the remainder isn't 0, divide the remainder into the divisor:
374 ÷ 264 = 1, with a remainder of 110.
Since the remainder still isn't 0, divide the remainder into the divisor:
264 ÷ 110 = 2, with a remainder of 44.
Since the remainder still isn't 0, divide the remainder into the divisor:
110 ÷ 44 = 2, with a remainder of 22.
Since the remainder still isn't 0, divide the remainder into the divisor:
44 ÷ 22 = 2, with a remainder of 0.
Now, when the remainder is 0, you are done and your answer (the greatest common factor) is the last divisor; so the greatest common factor is 22.