If a = i + j + k, b = j - k and c be three vectors such that a × c = b and a · c =3, then c ·( a -2 b ) is equal to ____.

If $\displaystyle \vec{a}=\hat{i}+\hat{j}+\hat{k}, \vec{b}=\hat{j}-\hat{k}$ and $\displaystyle \vec{c}$ be three vectors such that $\displaystyle \vec{a} \times \vec{c}=\vec{b}$ and $\displaystyle \vec{a} \cdot \vec{c}=3$, then $\displaystyle \vec{c} \cdot(\vec{a}-2 \vec{b})$ is equal to $\displaystyle \_\_\_\_$.