Answer
Because you can easily find the Kb of a base, using the Ka of his conjugate acid, with this formula:
$K_a * K_b = K_w = 10^{-14}$
Work Step by Step
If we took the Ka of phenol:
$Ka = 1.3 \times 10^{-10}$
$Ka * Kb = 10^{-14}$
$1.3 \times 10^{-10} * Kb = 10^{-14}$
$Kb = \frac{10^{-14}}{1.3 \times 10^{-10}}$
$Kb = 7.7 \times 10^{-5}$
We will find that the Kb of the phenolate is $7.7 \times 10^{-5}$
