If a and b are two vectors such that | a | = 1, | b | = 4 a . b = 2. If c = (2a $$\times$$ b) $$-$$ 3b, then angle between b and c

If $$a = - \widehat i + \widehat j + \widehat k$$ and $$b = 2\widehat i + \widehat k$$, then find z component of a vector r, which is coplanar with a and b, r . b = 0 and r . a = 7.