Answer
$|U|=13.$
Work Step by Step
The magnitude of a vector is given by the possible square root of the sum of the squares of its horizontal and vertical components.
We have $U=5i+12j.$
Then \begin{align*}
|U|&=\sqrt{(5)^{2}+(12)^{2}}\\
&=\sqrt{25+144}\\
&=\sqrt{169}\\
&=13.
\end{align*}
Hence, $|U|=13.$