Answer:
6
Explanation:
first, look at the first two coefficents, (12,6) 6 can divide 12 and 6 obviously didvides itself so we can day the gcd(12,6) is 6. now look at the last two coefficents (6,18). Similarly we can see that the gcd here is also 6.
finally, look at 12 and 18, neither divide each other so we can try dividing them by our most common gcd so far 6. this yields 2,3. niehter can be simplified further so we can conlcude the gcd of this monomials is 6