You're adding the two angles together to get to 126°, so let's start off simple:
∠ABC + ∠DBC = 126°
Since we know that ∠ABC = 4x + 8, we can replace ∠ABC in our equation with the expression 4x + 8. We can do the same with ∠DBC and the expression 6x + 4. Now, our equation looks more like this:
4x + 8 + 6x + 4 = 126
This equation looks a lot like answer B, so I'd go with that one!