Chapter 7 - Section 7.5 - Vectors: An Algebraic Approach - 7.5 Problem Set - Page 404: 32

$|U|=\sqrt{841}.$

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, $|U|=\sqrt{841}.$