If $ | \vec{a} | = 3 $, $ | \vec{b} | = 2 $, then find $ (3\vec{a} - 2\vec{b}) \cdot (3\vec{a} + 2\vec{b}) $.
Benzene diazonium chloride on treatment with reagent 'X' gives iodobenzene. The reagent ‘X’ is