(2x+3y)(2x-3y)(9y^2+4x^2)-(4x^2-9y^2)^2
=(4x^2-9y^2)(9y^2+4x^2)-(4x^2-9y^2)^2
=(4x^2-9y^2)(9y^2+4x^2+9y^2-4x^2)
=18y^2*(4x^2-9y^2)
(2x+3y)(2x-3y)(9y^2+4x^2)-(4x^2-9y^2)^2
=(4x^2-9y^2)(9y^2+4x^2)-(4x^2-9y^2)^2
=(4x^2-9y^2)(9y^2+4x^2+9y^2-4x^2)
=18y^2*(4x^2-9y^2)