${\vec{v}}_{21}=-2{\vec{v}}_1-{\vec{v}}_1$

${\vec{v}}_{21}=-3{\vec{v}}_1$

$v_{21}=\left|{\vec{v}}_{21}\right|$

$v_{21}=3v_1$

${\vec{v}}_{12}={\vec{v}}_1-\left(-2{\vec{v}}_1\right)$

${\vec{v}}_{12}=3{\vec{v}}_1$

$v_{12}=\left|{\vec{v}}_{12}\right|$

$v_{12}=3v_1$