Well I don't know !
Let's get together and figure it out.
The factors of 14a are
1, 1a, 2, 2a, 7, 7a, 14, and 14a .
The factors of 28ab are
1, 1a, 1ab, 2, 2a, 2ab, 4, 4a, 4ab,
7, 7a, 7ab, 14, 14a, 14ab, 28, 28a, and 28ab .
The common factors of 14a and 28ab are:
1, 1a, 2, 2a, 7, 7a, 14, and 14a .
It looks like 14a is their greatest common factor.
I don't know what happens if 'a' is less than ' 1 '.
This bothers me, but I'll let it go for now.