If you mean "what is the GCF of 3x + 18?," 3x + 18 is only one thing, so the GCF is just 3x + 18.
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If you mean "what is the GCF of 3x and 18?" then:
3x = 3 * x
18 = 2 * 3 * 3
The GCF is made of the factors that appear in both 3x and 18. In this case that's just 3, so the GCF is 3.
Answer: The GCF of 3x and 18 is 3.