Answer
True by axioms 1 and 3
Work Step by Step
$\langle\vec{u}, c\vec{v}\rangle=\langle c\vec{v}, \vec{u}\rangle$ by axiom 1
$\langle c\vec{v}, \vec{u}\rangle=c\langle \vec{v}, \vec{u}\rangle$ by axiom 3
$c\langle \vec{v}, \vec{u}\rangle=c\langle \vec{u}, \vec{v}\rangle$ by axiom 1
