Sorry to interfere here but the question is missing an important piece of information. It asks for the exact answer meaning a unique answer but it doesn't define in which quadrant the angle lies. Therefore there are an infinite number of solutions eg -5pi/6 , 7pi/6 etc. if one wants to be pedantic. In the end it's the student who suffers if they are not aware of such details.
+36915 ...but you have to select from the multiple choice answers given...hence the correct answer is - pi/6
