How do you obtain an answer in radians from a special triangle? For example how would you get the answer for this question? (The answer is written but I don't know how to get it). So far I have used the CAST method so I know it is in quadrants 3 and 4. I also know that it is a 30 degree triangle.

sinx= -1/2 Domain: 0<x<2(pi)

x=7(pi)/6, 11(pi)/6

Thanks