#### Answer

$+5$

#### Work Step by Step

The leading coefficient of the given trinomial is 1.
Thus, it can be factored by looking for the factors of the constant term (15) whose sum is equal to the numerical coefficient of the middle term (8).
Note that $15=3(5)$ and $3+5=8$.
Thus, the given trinomial can be factored as $(x+3)(x+5)$.
Therefore the term of the second factor is $+5$.