Answer
$(a+b)(x+5)(x-4)$
Work Step by Step
First, there is a GCF (of all terms), GCF=$(a+b)$
$...=(a+b)(x^{2}+x-20)$
Searching for two integer factors of $-20$ such that their sum is $+1...$
we find $5$ and $-4.$
= $(a+b)(x+5)(x-4)$
All are prime, factorization is complete