What is the equilibrium constant for the reaction of ammonium ion with formate ion?

Question

I know that the ionization constant for ammonium ion is Ka=5.6e-10 and for formate Kb=5.6e-11.

a) 1.0 X 10-13
b) 5.6 X 10-11
c) 3.1 X 10-6
d) 1.8 X 10-5
e) 3.2 X 105
I don't know what to do? Do I multiply them to get kw?? please help