Answer
$x^2+2xy+y^2-9$
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: $[(x+y)+3][(x+y)-3]=(x+y)^2-(3)^2=(x)^2+2\cdot x \cdot y+y^2-9=x^2+2xy+y^2-9$
