#### Answer

The expression can not be factored because there are no factors of $18$ with a sum of $15$.

#### Work Step by Step

Attempting to factor the trinomial, we search for
two factors of $ac=2\cdot 9=18$,
whose sum is $b=15$
Factors of 18:$\quad 1,2,3,6,9,18$
6 and 9 add to 15, but $6\cdot 9\neq 18.$
The expression cannot be factored because there are no factors of $18$ with a sum of $15$.