d
\(\begin{array}{l}
X = at + b{t^2} - c{t^3}\\
V = \frac{{dx}}{{dt}} = a + 2bt - 3c{t^2}\\
a = \frac{{dv}}{{dt}} = 2b - 6ct\\
Put\,acceleration\, = 0\\
\Rightarrow \,\,t = \frac{b}{{3c}}\\
Find\,V\,at\,t = \frac{b}{{3c}}\\
V = \,a + \frac{{{b^2}}}{{3c}}
\end{array}\)