4cos^2x+sinxcosx+3sin^2-3=0

4cos²x + sinx * cosx + 3sin²x - 3 = 0
 4cos²x + sinx * cosx + 3sin²x - 3(sin²x + cos²x) = 0
4cos²x + sinx * cosx + 3sin²x - 3sin²x - 3cos²x = 0
cos²x + sinx * cosx = 0
 cosx * (cosx + sinx)=0
cosx = 0 
  x=π/2 + πn,n ∈ Z
cosx + sinx = 0 |:cosx 
1+ tgx = 0
 tgx = -1
  x=-π/4 + πn,n ∈