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$D^2 [\sin(\theta)+\cos^3(\theta)]$

The answer should be $-\sin(\theta)+6\sin^2 (\theta)\cos(\theta)-3\cos^3 (\theta)$

I understand it's applying $D$ twice, but I can't tell which rules to use. Since it's addition I can do $D[\sin(\theta)]+D[\cos^3 (\theta)]$ $= [\cos(\theta)]+3[-\sin^{3-1}(\theta)]$ $= \cos(\theta)-3\sin^2(\theta)$ Then $D^2=-\sin(\theta)-6\sin(\theta)$

What the hell am I missing? Should I apply the chain rule somewhere in there?

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