In this problem, we want to determine which variables will give us the constant term of the trinomial.
We are given the binomials

When mutliplying these binomials, we can use the distributive property, or we can use the FOIL method.
Using distribution, we get:

Distributing the x and p for each term, we have

Recall that a constant term is any number that does not have a variable "attached" to it. For example, these are constants:

However, this is an abstract example. So for our purposes, the only true variable will be x, while p and q represent some unknown real numbers.
Since x is our variable, we see only the last term has no x attached to it.

Therefore, the constant term of the trinomial is the product of -p and q.