well the GCF is the number you can get out of your terms that can later be multiplied and get the same answer... in this case lets chack our smallest number that fits in our smallest numbers of these three wellit will be 3 how many times does 3 fit in 9?? 3 times, how many times does it fir in 219?? 73 times, and how many times does it fir in 21?? well it fits 7 times now lets set it as a factor 3(3+73+7) now is there any other number that can fir on the ones inside the parenthesis?? Well no so your final answer should be 3(3+73+7)
hope this helps