The sum of the solutions \( x \in \mathbb{R} \) of the equation \( \dfrac{3\cos 2x + \cos^{3} 2x}{\cos^{6} x - \sin^{6} x} = x^{3} - x^{2} + 6 \) is:
\( 1 \)
\( 0 \)
\( -1 \)
\( 3 \)