Question 8

Prove that the points whose position vectors are $A(3\mathbf{i} + 2\mathbf{j} - \mathbf{k})$, $B(\mathbf{i} - 2\mathbf{j} + \mathbf{k})$, $C(6\mathbf{i} + 4\mathbf{j} - 2\mathbf{k})$, $D(9\mathbf{i} + 6\mathbf{j} - 3\mathbf{k})$ are coplanar.