From Mechanics

\displaystyle \begin{align} & \mathbf{v}=\frac{d\mathbf{r}}{dt}=(-4t)\mathbf{i}+(-10t)\mathbf{j} \\ & \mathbf{v}(2)=-8\mathbf{i}-20\mathbf{j} \\ & v(2)=\sqrt{{{8}^{2}}+{{20}^{2}}}=21\textrm{.}5\text{ m}{{\text{s}}^{\text{-1}}} \\ \end{align}