You need two things to find something in common.
3x
3X
3x
3x
The GCF is x5
Since 3x is a factor of 3x^2, it is automatically the GCF.
The GCF is 3x
The GCF of 6x and 9x is 3x
The Greatest Common Factor is 3.
3x
5(3x-7)
The greatest common factor is 6x.