I always find it easier to solve problems like this by substituting x for sin theta
So our equation is 2x^2 -3x +1 =0. This factors to
(2x-1)(x-1)=0
x= 1/2. x=1 which gives sin theta=1/2 and sint theta= 1
The angles that have a sin equal to 1/2 are pie/6 and 5pie/6
The angle with a sin of 1 is pie/2 so our solutions are
pie/6 +2pie(n)
pie/2 +2pie(n)
5pie/6 +2pie(n)