Answer
$(x-3-y)(x-3+y)$
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: $x^2-6x+9-y^2=x^2-2\cdot3\cdot x+3^2-y^2=\\=(x-3)^2-y^2\\=(x-3-y)(x-3+y)$