Is 28 a common factor of 7 and 2?

CAREFUL! 28 is a common multiple of 7 and 2!

The common factors of 7 are:

1 x 7

-1 x -7

The common factors of 2 are:

1 x 2

-1 x -2

So, as you can see, the common factors of 7 and 2 are 1and -1