Answer:
III quadrant
Step-by-step explanation:
First of all, let's write the components of each vector along the two directions. We have:

where we put a negative sign on both components, since they are in the negative x- and y- direction.
Similarly,

where we put a negative sign on the y-direction, since it is along the negative direction.
Now we sum the components along each axis:

We see that both components of (B+C) are negative, so this vector must be in the 3rd quadrant.