Answer
(a) $ \sin 3 x+\cos 3 x$ is a solution.
(b) $3 \sin x+3 \cos x$ is not a solution.
(c) $\sin 3x$ is a solution.
(d) $\cos 3x$ is a solution.
Work Step by Step
(a) $ \sin 3 x+\cos 3 x$ is a solution.
(b) $3 \sin x+3 \cos x$ is not a solution.
(c) $\sin 3x$ is a solution.
(d) $\cos 3x$ is a solution.