From Mechanics

Using the result of part b) with \displaystyle a=0 gives

\displaystyle \begin{align} & 2t-\frac{3{{t}^{2}}}{20}=0 \\ & \text{which gives } \\ & t=0\ \text{s }\ \text{or }\ t=\frac{40}{3}=13\textrm{.}3\ \text{s} \\ \end{align}