7. For the displacement field of \( \mathbf{u}=\left(X_{1}-X_{3}\right)^{2} \mathbf{e}_{1}+\left(X_{2}+X_{3}\right)^{2} \mathbf{e}_{2}-X_{1} X_{2} \mathbf{e}_{3} \), determine the change in length per unit length (normal strain) in the direction of \( \mathbf{v}=\left(8 \mathbf{e}_{1}-\mathbf{e}_{2}+4 \mathbf{e}_{3}\right) / 9 \) at point \( P(0,2,-1) \).