I need a little help here !!!

Find the following values exactly:

a) sin (7pi/6) I got -1/2 but i don't know how i got it please someone explain

b) sin (-11pi/4)

2) Find the general soln of the trig eqn

3cosx = 2sin^2x

My answer is : (2 cos x - 1) (cos x + 2) is that right ?