Hey there! :)
I believe the correct answer to your question is 12.
Find the prime factorization of 84
84 = 2 × 2 × 3 × 7
Find the prime factorization of 108
108 = 2 × 2 × 3 × 3 × 3
To find the gcf, multiply all the prime factors common to both numbers:
Therefore, GCF = 2 × 2 × 3
GCF = 12
Hopefully this can help you out!