Answer
$(x+3-y)(x+3+y)$
Work Step by Step
Using factoring techniques, then,
\begin{array}{l}
x^2+6x+9-y^2
\\=
(x^2+6x+9)-y^2
\\=
(x+3)^2-y^2
\\=
[(x+3)-y][(x+3)+y]
\\=
(x+3-y)(x+3+y)
.\end{array}
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