Answer
$[OH^-] = 2.5 \times 10^{- 7}M$
Work Step by Step
1. Use "$K_w$" to calculate the $[OH^-]$
$[H_3O^+] * [OH^-] = Kw = 10^{-14}$
$ 4 \times 10^{- 8} * [OH^-] = 10^{-14}$
$[OH^-] = \frac{10^{-14}}{ 4 \times 10^{- 8}}$
$[OH^-] = 2.5 \times 10^{- 7}M$