From Mechanics

Here we have \displaystyle \mathbf{a}=0\textrm{.}2\mathbf{i}+0\textrm{.}3\mathbf{j},\ \ \displaystyle \mathbf{u}=4\mathbf{i}– 8\mathbf{j}\ \ and \displaystyle \ \ \mathbf{ r}_{0}=6\mathbf{i}+2\mathbf{j}.

Using \displaystyle \mathbf{v}=\mathbf{u}+\mathbf{a}t\ \ at \displaystyle \ \ t=10 gives

\displaystyle \mathbf{v}=4\mathbf{i}– 8\mathbf{j}+(0\textrm{.}2\mathbf{i}+0\textrm{.}3\mathbf{j})\times 10\ \ or

\displaystyle \mathbf{v}=6\mathbf{i}– 5\mathbf{j}\ \ \text{m}{{\text{s}}^{-1}}

The speed is the magnitude of this velocity vector.

\displaystyle v=\sqrt{{{6}^{2}}+{{\left( -5 \right)}^{2}}}=\sqrt{61}=7\textrm{.}8\ \text{m}{{\text{s}}^{-1}}