Answer
Please see the work below.
Work Step by Step
a. We know that
$K_b=\frac{K_w}{K_a}$
$K_b=\frac{1.0\times 10^{-4}}{6.8\times 10^{-14}}=1.47\times 10^{-11}$
b. We know that
$K_a=\frac{K_w}{K_b}$
$K_a=\frac{1.0\times 10^{-12}}{4.2\times 10^{-10}}=2.38\times 10^{-5}$