If z = (3 + 7i) (a + ib) where a,b∈Z−{0}, is purely imaginary, then the minimum value of |z|2 is

z=(3a−7b)+i(3b+7a)For z to be purely imaginary we have3a=7b⇒a=7 or b=3            (for least value of |z|)⇒|z|=9+49=58