Answer
$(y+12)(y+3)$
Work Step by Step
Using the factoring of trinomials in the form $x^2+bx+c,$ the given $\text{
expression
}$
\begin{align*}
y^2+15y+36
\end{align*} has $c=
36
$ and $b=
15
.$
The two numbers with a product of $c$ and a sum of $b$ are $\left\{
12,3
\right\}.$ Using these two numbers, the factored form of the $\text{
expression
}$ above is
\begin{align*}
&
(y+12)(y+3)
\end{align*}