(x+y+4)(x+y-4)=(x+y)^2-16=x^2+y^2+2xy-16
(2x+3)^2*(2x-3)^2=(4x^2-9)^2=16x^4-72x^2+81
(x-3)(x+3)(x^2+9)=(x^2-9)(x^2+9)=x^4-81