(a)[1]
Show that $f(x)$ has $x + 7$ as a factor.
(b)[2]
Determine the quotient obtained when $f(x)$ is divided by $x + 7$.
(c)[3]
Hence solve the equation $8\cos^3\theta + 54\cos^2\theta - 17\cos\theta - 21 = 0$, with $0^\circ \leq \theta \leq 360^\circ$.