Answer
$ \sqrt 5$
Work Step by Step
Let us consider two vectors $v=pi+qj$ and $w=xi+yj$; then we have: $v \pm w=(p \pm x)i+(q \pm y)j$
The magnitude of any vector (let us say $v$) can be determined using the formula
$||v||=\sqrt{p^2+q^2} $
Therefore, $||v||=\sqrt{(-2)^2+(1)^2} \\
= \sqrt {4+1} \\
= \sqrt 5$
