2sin y = tan y
2sin y - sin y / cos y = 0
sin y (2cos y - 1) = 0
either siny = 0 or cos y = 1/2
From sin y = 0, you get your 180 degrees.
Remember for such equations, never divide the sin y away. You
will lose solutions, as shown clearly by your example.
Another example would be 2x^2 - x = 0
You don't divide by x to get 2x -1 = 0. Instead, you factorise to x
(2x - 1) = 0. The same thing is to be used on your question
here.
Regards,
Eagle
Owner of Strategic Tuition