You want to make some reference to an angle either in quadrant 2 or quadrant 4. You could ignore staying strictly to a unit-circle and just look at the sides of a right triangle. Imagine an angle pointing from origin into quadrant 4, because this will be easier to draw or visualize.
To continue with r, the hypotenuse for the triangle
This again would be for a angle x LESS THAN 0 measure, and going into quadrant 4.
Taking , this is . ---------------------which will be 30 degrees, or .
Actually, for the original equation according to the assumption made, the actual answer can be EITHER OR .