Let $\vec{a}=\vec{i}+2\hat{j}+4\hat{k},\hat{b}=\hat{i}+\lambda \hat{j}+4\hat{k}$ and $\vec{c}=2\hat{i}+4\hat{j}+\left({\lambda }^2-1\right)\hat{k}$ be coplanar vectors. Then the

non-zero vector  $\vec{a}\times \vec{c}$  is :