Answer
Please see the work below.
Work Step by Step
We know that
$K_b=\frac{K_w}{K_a}=\frac{1.0\times 10^{-14}}{4.9\times 10^{-10}}=2.04\times 10^{-5}$
As $K_b=\frac{K_w}{K_a}$
$K_b=\frac{1.0\times 10^{-14}}{4.8\times 10^{-11}}=2.8\times 10^{-4}$
