Suppose $\vec{u} = \left<2,1,-3\right>$. Then

$\left<6,0,4\right>$ makes		? an obtuse angle a right angle an acute angle with $\vec{u}$
$\left<-3,4,3\right>$ makes		? an obtuse angle a right angle an acute angle with $\vec{u}$
$\left<3,5,-3\right>$ makes		? an obtuse angle a right angle an acute angle with $\vec{u}$
$\left<3,-3,2\right>$ makes		? an obtuse angle a right angle an acute angle with $\vec{u}$