$ \lim _{x \rightarrow 0} \frac{f(x)}{x^3} $
Using L Hopital Rule.
$ \lim _{x \rightarrow 0} \frac{f^{\prime}(x)}{3 x^2}=\lim _{x \rightarrow 0} \frac{x+\sin \left(1-e^x\right)}{3 x^2} \text { (Again L Hopital) } $
Using L.H. Rule
$ \begin{aligned} & =\lim _{x \rightarrow 0} \frac{-\left[\sin \left(1-e^x\right)\left(-e^x\right) \cdot e^x+\cos \left(1-e^x\right) \cdot e^x\right]}{6} \\ & =-\frac{1}{6} \end{aligned} $