Vector Algebra

Let  $\overrightarrow a = \widehat i + 2\widehat j + 4\widehat k,$ $\overrightarrow b = \widehat i + \lambda \widehat j + 4\widehat k$ and $\overrightarrow c = 2\widehat i + 4\widehat j + \left( {{\lambda ^2} - 1} \right)\widehat k$ be coplanar vectors. Then the non-zero vector $\overrightarrow a \times \overrightarrow c$ is :