Solution of differential equation using Laplace transform Can someone please help me identify my mistake as I am not able to find it.
$y''+y'=3x^2$ where $y(0)=0,y'(0)=1$
$L[y'']+L[y']=3L[x^2]$
$L[y'']=p^2L[y]-py(0)-y'(0)=p^2L[y]-1$
$L[y']=pL[y]-y(0)=pL[y]$
$p^2L[y]-1+pL[y]=\frac{6}{p^3}$
$p^2L[y]+pL[y]=\frac{6}{p^3}+1$
$(p^2+p)L[y]=\frac{6}{p^3}+1$
$L[y]=\frac{6}{p^3(p^2+p)}+\frac{1}{p^2+p}$
$y=x^3-e^{-x}+1-e^{-x}$

Using 
\begin{align}
\mathcal{L}\{y'\} &= s \, \overline{y} - y(0) \\
\mathcal{L}\{y''\} &= s^2 \, \overline{y} - s \, y(0) - y'(0) 
\end{align}
then
$$y'' + y' = a \, t^2$$
transforms to
\begin{align}
s \, (s+1) \, \overline{y} &= s \, y(0) + y'(0) + y(0) + \frac{2 a}{s^3} \\
\overline{y} &= \frac{y(0) + y'(0)}{s \, (s+1)} + \frac{y(0)}{s+1} + \frac{2 a}{s^4 \, (s+1)} \\
&= \frac{y(0) + y'(0) - 2 a}{s} + \frac{2 a - y'(0)}{s+1} + \frac{2 a}{s^4} - \frac{2 a}{s^3} + \frac{2 a}{s^2} \\
&= \mathcal{L}\{ (2a - y'(0)) \, e^{-t} + \frac{a}{3} \, t^3 - a \, t^2 + 2 a \, t + y(0) + y'(0) - 2 a  \}.  
\end{align}
This leads to the general solution
$$y(t) = (2a - y'(0)) \, e^{-t} + \frac{a}{3} \, t^3 - a \, t^2 + 2 a \, t + y(0) + y'(0) - 2 a. $$
Now applying the conditions $y(0) = 0$, $y'(0) =1$, $a=3$ then
$$y(t) = 5 \, e^{-t} + t^3 - 3 \, t^2 + 6 \, t - 5. $$
Note that a key component is 
$$\frac{1}{s^4 \, (s+1)} = \frac{1}{s+1} + \frac{1}{s^4} - \frac{1}{s^3} + \frac{1}{s^2} - \frac{1}{s}.$$  
