What Is the Value of K_b for the Formate Anion,HCOO^- $\times$

What is the value of K_b for the formate anion,HCOO^-?
K_a(HCOOH) = 2.1 $\times$ 10^-4

A) -2.1 $\times$ 10^-4
B) 2.1 $\times$ 10^-4
C) 6.9 $\times$ 10^-6
D) 4.8 $\times$ 10^-11
E) 2.1 $\times$ 10^-18