Answer
$(y+3)(3y+5)$
Work Step by Step
The 2 numbers whose product is $ac=
3(15)=45
$ and whose sum is $b=
14
$ are $\left\{
9,5
\right\}$. Using these numbers to decompose the middle term results to
\begin{array}{l}
3y^2+9y+5y+15
\\\\=
(3y^2+9y)+(5y+15)
\\\\=
3y(y+3)+5(y+3)
\\\\=
(y+3)(3y+5)
.\end{array}
