The GCF(or greatest common factor) is the maximum number or variable or both that you can get out of a polynomial, in this case your GCF would be 2 because it can fit in all of your numbers now that we know that lets check what my answers would be so what do I need to multiply by 2 in order to get 4k?? well it will be 2k now do the same with the others. in order to get 18k4 I have to multiply 2 times 9k4 and in order to get 12 I have to multiply 2 times 6, now lets set our numbers.. It should look like 2(2k+9k4+6) now our final step is to check if it can be simplified even more.. In this case you can't so your answer would simple be that the GCF is 2
Hope this helps