Answer
$(x+3)(x+5)$
Work Step by Step
Using factoring by grouping, the given expression, $
x^2+3x+5x+15
,$ is equivalent to
\begin{array}{l}\require{cancel}
(x^2+3x)+(5x+15)
\\\\=
x(x+3)+5(5+3)
\\\\=
(x+3)(x+5)
.\end{array}
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