what is the Kb value for the base?

given:

0.120M weak base

pH=11.28

initially i thought about the equation

pH=pKa+log[base]

[acid]

so i did:

11.28=pKa+log(0.120)

pKa=12.20

pKa=-log Ka

10^{-12.20}

Ka=6.31*10^{-13}

Ka*Kb=Kw=1.0*10^{-14}

Kb=.0158

this is not the answer, however.

please help me determine the answer