Divide both sides by cos2x:
Tan2x+3=0
Tan2x=-3
2tanx/1-tan2x=-3
2tanx=-3+3tan2x
3tan2x-2tanx-3=0
Tanx= 1.387, or tanx= -0.720
Therefore, x= 54.2, -35.75
Now use the tan graph or the ASTC rule to find the other angles possible:
X= 54.2, 234.2 -35.75, 144.5
Therefore, x= 54.2, 144.5