Answer
$(y-3)^2$
Work Step by Step
Using the factoring of trinomials in the form $x^2+bx+c,$ the given $\text{
expression
}$
\begin{align*}
y^2-6y+9
\end{align*} has $c=
9
$ and $b=
-6
.$
The two numbers with a product of $c$ and a sum of $b$ are $\left\{
-3,-3
\right\}.$ Using these two numbers, the factored form of the $\text{
expression
}$ above is
\begin{align*}
&
(y-3)(y-3)
\\&=
(y-3)^2
\end{align*}