Is this meant to be: 4tan2(x)-1=tan2(x)? If so:
3tan2(x)=1, tan(x)=1/√3, x=π/6 (30°). Also, π+π/6=7π/6 (210°). (Most likely answer.)
If the question is meant to be: 4tan(x2)-1=tan2(x) then the answer is much more complicated.
Graphically x has numerous values between 0 and 2π (approximate radians):
0.589, 2.018, 2.573, 3.110, 3.587, 4.034, 4.499, 4.847, 5.114, 5.372, 5.638, 5.903, 6.160. (This is an unlikely solution.)