3sin^2x -2sinxcosx -cos^2x=0

[latex]3tg^2x-2tgx-1=0 \\ tgx=1 \\ tgx=- \frac{1}{3} \\ \\ x= \frac{ \pi }{4} + \pi n \\ x=-arctg \frac{1}{3} + \pi k, \\ n,k \in Z [/latex]