Cos^2*6x-sin^2*3x-1=0

[latex]cos^26x-sin^23x-1=0\\\\\boxed{sin^2\alpha=\frac{1-cos2\alpha}2}\\\\cos^26x-\frac{1-cos6x}2-1=0\\\\2cos^26x-1+cos6x-2=0\\\\2cos^26x+cos6x-3=0\\\\D=1+24=25\\\\cos6x=\frac{-1\pm5}4\\\\cos6x=1 \ || \ cos6x=\frac{-6}4 \ (<-1)\\\\6x=2\pi n\\\\\boxed{x=\frac{\pi n}3, \ n\in Z}[/latex]