d
\(\begin{array}{l}
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Give\,:\,x = 8 + 12t - {t^3}\\
\,velocity\,,\,v = \frac{{dx}}{{dt}} = 12 - 3{t^2}\\
When\,v = 0,\,12 - 3{t^2} = 0\,\,\,or\,\,t = 2\,s\\
\,\,\,\,\,\,\,\,\,\,\,a = \frac{{dv}}{{dt}} = - 6t\\
\,\,\,\,\,\,\,\,\,\,\,a\left| {_{t = 2\,s} = - 12\,m\,{s^{ - 2}}} \right.\\
{\rm{Retardation}}\, = 12\,m\,{s^{ - 2}}
\end{array}\)