what is the greatest common factor of the terms of 90x^6y^6+18x^4y^4+27x^2y^2
check all the powers. Th highest power present for each variable will be in the GCF. (common factor, right?)
So, you have
90x^6y^6
18x^4y^4
27x^2y^2
The biggest thing common to all those terms is
9x^2y^2