${\vec{v}}_{21}=2{\vec{v}}_1-\left(-{\vec{v}}_1\right)$

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

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

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

$v_{21}=3v_1$

${\vec{v}}_{12}=-{\vec{v}}_1-2{\vec{v}}_1$

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

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

$v_{12}=3v_1$