b
\(\begin{array}{l}
V = b\sqrt X \\
\frac{{dv}}{{dt}} = \frac{b}{{2\sqrt X }}\frac{{dx}}{{dt}}\,;\,a = \,\frac{{bv}}{{2\sqrt X }}\\
a = \frac{{b\left( {b\sqrt X } \right)}}{{2\sqrt X }}\,;\,\frac{{dv}}{{dt}} = a = \frac{{{b^2}}}{2}\,;\,v = \frac{{{b^2}}}{2}\tau
\end{array}\)