There cannot be a greatest common factor (GCF) of just one number. To be common there need to be at least two numbers. If you find all the factors of two or more numbers, and you find some factors are the same ("common"), then the largest of those common factors is the Greatest Common Factor.
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The Greatest Common Factor (GCF) is: 13
39 and 93's highest common factor is three.
39 and 45's highest common factor is three.
It is: 13
The GCF is 13.