5.9. For what values of $x$ are the vectors $(x^{3}-1) \bar{a}$ and $2 x \bar{a}$ collinear, if $\bar{a} \neq \overline{0}$?
5.9. The vectors are collinear if \(x^{3}-1\) and \(2 x\) have the same sign. To determine the desired values of \(x\), we solve the inequality \(2 x\left(x^{3}-1\right)>0\) and obtain \(x \in(-\infty, 0) \bigcup(1, \infty)\).

Answer: for \(-\infty<x<0\) and for \(1<x<\infty\).