Answer
$(x+5+y)(x+5-y)$
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: $x^2+10x-y^2+25=\\=x^2+2\cdot x\cdot5+5^2-y^2\\=(x+5)^2-y^2\\=(x+5+y)(x+5-y)$
