From Mechanics

\displaystyle \begin{align} & \mathbf{v}=\frac{d\mathbf{r}}{dt}=\left( 2-\frac{t}{5} \right)\mathbf{i}+2\mathbf{j} \\ & v=\sqrt{{{\left( 2-\frac{t}{5} \right)}^{2}}+{{2}^{2}}}=\sqrt{8-\frac{4t}{5}+\frac{{{t}^{2}}}{25}} \\ \end{align}