Answer
$7$
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
$||v||=\sqrt{3^2+(-6)^2+(-2)^2}\\
||v||=\sqrt{9+36+4}\\
||v||=\sqrt{49}\\
||v||=7$