The numerical coefficients of the given monomials are 36, 54 and 90. The greatest common factor of 36, 54 and 90 is 18.The common literals appearing in the three monomials are a and c. The smallest power of a in the three monomials is 2.The smallest power of c in the three monomials is 2.The monomial of common literals with the smallest powers is a2c2.​Hence, the greatest common factor is 18a2c2.