#### Answer

$(3a-5)^2$

#### Work Step by Step

The two numbers whose product is $ac=
9(25)=225
$ and whose sum is $b=
-30
$ are $\{
-15,-15
\}$. Using these two numbers to decompose the middle term, then the factored form of the expression, $
9a^2-30a+25
$, is
\begin{array}{l}\require{cancel}
9a^2-15a-15a+25
\\\\=
(9a^2-15a)-(15a-25)
\\\\=
3a(3a-5)-5(3a-5)
\\\\=
(3a-5)(3a-5)
\\\\=
(3a-5)^2
.\end{array}