#### Answer

$(y-3)^2$

#### Work Step by Step

Using the factoring of trinomials in the form $x^2+bx+c,$ the given expression, $
y^2-6y+9
,$ has $c$ equal to $
9
$ and $b$ equal to $
-6
.$
The two numbers that give a product of $c$ and a sum of $b$ are $\{
-3,-3
\}.$ Hence, the factored form of the expression above is
\begin{array}{l}\require{cancel}
(y-3)(y-3)
\\\\=
(y-3)^2
.\end{array}