The numerical coefficients of given numerical are 4, -12, 18.

Greatest common factor of 4, -12, 18 is 2.

Common literals appearing in given numerical are a and b

Smallest power of a in three monomials = 2

Smallest power of b in three monomials = 1

Monomials of common literals with smallest power= a²b

Hence, the greatest common factor = 2a²b