With zero initial state, $H(s)=3/(s+1/6)$ and $X(s)=3/(s+1/3)$. Hence $Y(s)=9/[(s+1/6)(s+1/3)]$.
Partial fractions give $Y=54/(s+1/6)-54/(s+1/3)$. Inverse transformation yields **$54(e^{-t/6}-e^{-t/3})u(t)$, D**. This also gives y(0+)=0 as required by the zero-state model.
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