Question:

If $\vec{i} \times [(\vec{a} - \vec{j}) \times \vec{i}] + \vec{j} \times [(\vec{a} - \vec{k}) \times \vec{j}] + \vec{k} \times [(\vec{a} - \vec{i}) \times \vec{k}] = \vec{0}$ and $\vec{a} = x\vec{i} + y\vec{j} + z\vec{k}$, then the value of $8(x^3 - 2y + 2z)$ is equal to