Take the square root of both sides, what do you find?
I see. The square root of 3/4 is, square root 3/2, which is Pi/3
Therefore as it is Positive in quadrants I and II the angle must be Pi - Pi/3 = 2Pi/3
so for + square root 3/2 the angles are Pi/3 and 2Pi/3
and for - square root 3/2 the angles are 4Pi/3 and 5Pi/3
just think like normal equation when u solve sin,tan and cos the only diffrent is kinda you have to take away or add pi to get to your intervall :P. Took me so long time to understand this and its so simple (I feel stupid that i did not understand this alot early <.<) btw i recomend to always draw unit circle