3 Two points \(A\) and \(B\) have coordinates ( \(3 a , - a\) ) and ( \(- a , 2 a\) ) respectively, where \(a\) is a positive constant.
- Find the equation of the line through the origin parallel to \(A B\).
- The length of the line \(A B\) is \(3 \frac { 1 } { 3 }\) units. Find the value of \(a\).