#### Answer

$(x+3)(x-5)$

#### Work Step by Step

Regroup the terms to obtain:
$=(x^2+3x)+(-5x-15)$
Factor out the greatest common factor of each group to obtain:
$=x(x+3) + (-5)(x+3)$
The $GCF$ for the two terms is a binomial, $(x+3)$. Even though it is a binomial, we can factor it out just like we would factor out a number or a variable. Thus, we factor out the binomial to obtain:
$=(x+3)[x+(-5)]
\\=(x+3)(x-5)$