Answer
$(a+b)(x-9)(x-4)$
Work Step by Step
First, there is a GCF (of all terms), GCF=$(a+b)$
$...=(a+b)(x^{2}-13x+36)$
Searching for two integer factors of $36$ such that their sum is $-13...$
we find $-9$ and $-4.$
= $(a+b)(x-9)(x-4)$
All are prime, factorization is complete