Answer
$9y^2(x+2)(x-2)$
Work Step by Step
The product of a binomial sum and a binomial difference is: $(A+B)(A-B)=A^2-B^2$.
The square of a binomial sum is: $(A+B)^2=A^2+2AB+B^2$.
The square of a binomial difference is: $(A-B)^2=A^2-2AB+B^2$.
Hence here: $9x^2y^2-36y^2=\\=9y^2(x^2-4)\\=9y^2(x^2-2^2)\\=9y^2(x+2)(x-2)$