Find the angle in Radians...

A circle with a radius of 1.

the point P giving the co-ordinates (-√3/2 , -1/2)

So it rotates through the 1st and 2nd quadrant and ends in the 3rd at point P. The angle in the third I'm unsure of. I know that its already 180 degrees.

would i solve the 3rd quadrant angle by finding it using cos θ A/H

so it would be (-√3/2)/1 = -1.225?

Thanks for any help.