As you said, in degrees what you have is:

2sin(x) -1 = 0 where 90< x < 180

So rearrange for sin(x):

sin(x) = 1/2

So find a value of x:

arcsin(x) = 30 degrees

Now sketch a graph of sin(x) and look the values of x for your range, where y = 0.5. You should see that x can also be 30 + 360 and 180 - 30 etc.

Now can you see the value of x for 90< x < 180?