Answer:
3
Explanation:
You can find the factors 15 and 51, and see what numbers they have in common.
51 = 1, 3, 17, 51
15 = 1, 3, 5, 15
If you look at it, the two numbers they have in common are 1 and 3. But the greatest out of the two is 3.
Answer: The greatest common factor (GCF) of 15 and 51 is 3
Hope this helps! :)