2sin^2 3x-2sin3x-4=0

$$\eqalign{
& 2{\sin ^2}3x – 2\sin 3x – 4 = 0 \cr
& \Leftrightarrow \left[ \matrix{
\sin 3x = 2\,\,\left( {loai} \right) \hfill \cr
\sin 3x = – 1 \hfill \cr} \right. \Leftrightarrow 3x = – {\pi \over 2} + k2\pi \cr
& \Leftrightarrow x = – {\pi \over 6} + {{k2\pi } \over 3}\,\,\left( {k \in Z} \right) \cr} $$