Vector Algebra

If $\mathbf{a}, \mathbf{b}, \mathbf{c}$ are unit vectors such that $\mathbf{a}$ is perpendicular to both $\mathbf{b}, \mathbf{c}$ and angle between $\mathbf{b}, \mathbf{c}$ is $2 \pi / 3$, then $|a+3 b-4 c|^2=$