Problem Statement
(a) Expand and simplify fully the expression $(a − 1)(6a^2 − a − 1)$.
(b) Determine all values of $\theta$ with $6 \cos^3\theta −7\cos^2\theta + 1 = 0$ and $−180^{\circ} < \theta < 180^{\circ}$.
(c) Determine all values of $\theta$ with $6 \cos^3\theta −7\cos^2\theta + 1 < 0$ and $−180^{\circ} < \theta < 180^{\circ}$.