Okay,

Let's see where cos is negative.

That would be the second and third quadrants.

Now let's calculate a reference angle.

Why did I drop the negative? Because we just determined where it will be negative. All we want now is a reference angle, which we will call .

Now substitute into the second and third quadrants.

Quadrant 2:

Quadrant 3:

I'm sure you can handle the rounding