3cos^2x-sin^2+2sinx*cosx=0

3cos²x-sin²x+2sinxcosx=0/cos²x≠0
tg²x-2tgx-3=0
tgx=a
a²-2a-3=0
a1+a2=2 U a1*a2=-3
a1=-1⇒tgx=-1⇒x=-π/4+πn,n∈Z
a2=3⇒tgx=3⇒x=arctg3+πk,k∈Z