Answer
$\sqrt{116}$
Work Step by Step
Here,
$\begin{align}
& a=2 \\
& b=-5 \\
& k=-2
\end{align}$
Therefore,
$\begin{align}
& -2\mathbf{u}=\left[ (-2)\times \left( 2 \right) \right]\mathbf{i}+\left[ (-2)\times \left( -5 \right) \right]\mathbf{j} \\
& =-4\mathbf{i}+10\mathbf{j}
\end{align}$
So,
$\begin{align}
& \left\| -2\mathbf{u} \right\|=\sqrt{{{\left( -4 \right)}^{2}}+{{10}^{2}}} \\
& =\sqrt{16+100} \\
& =\sqrt{116}
\end{align}$
