Answer
$\sqrt{3}.$
Work Step by Step
The magnitude $||v||$ of a vector $v=ai+bj+ck$, can be computed using the formula
$||v||=\sqrt{a^2+b^2+c^2}$.
Hence, using the formula above gives
$||v||=\sqrt{1^2+(-1)^2+1^2}\\||v||=\sqrt{1+1+1}\\||v||=\sqrt{3}.$
