Answer
See the answer below.
Work Step by Step
a) $K_b=K_w/K_a=4.35\cdot10^{-4}$
b) It would be between hydrogen carbonate and hexaaquanickel ion. Water is an acid weaker than the ion and hydroxide is a base stronger than the molecule.
c) $K_a=x\cdot x/(0.015-x)$
$x=[H_3O^+]=5.87\cdot10^{-7}$
$pH=6.23$
