Let a = i +2 j +4 k, b = i +λ j +4 k and c =2 i +4 j +(λ^2-1) k be coplanar vectors. Then the non-zero vector a × c is:

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