Answer
$(x-y-4)^2$
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: $(x-y)^2-8(x-y)+16=\\=(x-y)^2-2\cdot(x-y)\cdot4+4^2\\=(x-y-4)^2$
