Answer
$|U|=\sqrt{841}.$
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 $U=20i-21j.$
Then \begin{align*}
|U|&=\sqrt{(20)^{2}+(-21)^{2}}\\
&=\sqrt{400+441}\\
&=\sqrt{841}.
\end{align*}
Hence, $|U|=\sqrt{841}.$