\(f'(x) = -3x^2 + 2ax + a - 6 \leq 0 \\ \iff \Delta' = a^2 + 3(a - 6) \leq 0 \\ \iff a^2 + 3a - 18 \leq 0 \\ \iff -6 \leq a \leq 3\)