Answer
$(3x+y-5)(3x+y+5)$
Work Step by Step
Using $a^2-b^2=(a+b)(a-b)$ or the factoring of the difference of two squares, then,
\begin{array}{l}
(3x+y)^2-25
\\=
[(3x+y)-5][(3x+y)+5]
\\=
(3x+y-5)(3x+y+5)
.\end{array}
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