= $ - \frac{1}{2}\left| {\,\begin{array}{*{20}{c}}0&\omega &{{\omega ^2}}\\0&1&{ - 2}\\0&{ - 1}&0\end{array}\,} \right| = 0$, (Apply ${C_1} \to {C_1} + {C_2} + {C_3})$.