The answer is:

Work/explanation:
What does it mean to factor completely?
To factor an expression completely, we find its GCF, and factor it out.
Let's do it with the expression we have here:
.
I begin by finding the GCF. In this case, the GCF is 6x.
Next, I divide each term by -6x:

I end up with:

Hence, the factored expression is
.