Let b = i + j +λ k, λ ∈ R. If a is a vector such that a × b =13 i - j -4 k and a · b +21=0, then ( b - a ) ·( k - j )+( b + a ) ·( i - k ) is equal…

Let $\displaystyle \vec{b}=\hat{i}+\hat{j}+\lambda \hat{k}, \lambda \in \mathbb{R}$. If $\displaystyle \vec{a}$ is a vector such that $\displaystyle \vec{a} \times \vec{b}=13 \hat{i}-\hat{j}-4 \hat{k}$ and $\displaystyle \vec{a} \cdot \vec{b}+21=0$, then $\displaystyle (\vec{b}-\vec{a}) \cdot(\hat{k}-\hat{j})+(\vec{b}+\vec{a}) \cdot(\hat{i}-\hat{k})$ is equal to \_\_\_\_.