$1).$Given the points $A(-3,3,-6),B(-7,-9,-3),$ and $C(1,-1,7),$ find $($a$)$ the angle between vec$(AB)$ and vec$(AC),($b$)$ the angle between vec$(BA)$ and vec$(BC),$ and $($c$)$ the angle between vec$(CA)$ and vec$(CB).$

$($a$)\frac{?}{{?}_{B}AC}=*0$

$($b$)\frac{?}{{?}_{A}BC}=$

$($c$)\frac{?}{{?}_{A}CB}=$