Answer
$14$
Work Step by Step
Let us consider: $v=pi+qj+rz$
The magnitude of a vector can be determined using the formula
$||v||=\sqrt{p^2+q^2+r^2} ...(1)$
We will use the formula (1) to obtain:
$||v||=\sqrt{(-6)^2+(12)^2+(4)^2}\\
=\sqrt{36+144+16}\\
=\sqrt{196}\\
=14$