Answer
$[OH^-]=0.022 M$
$pH=12.34$
Work Step by Step
$[OH^-]=x$
$K_b=6.4\times10^{-4}=\frac{x\times x}{0.075-x}$
We can assume that
$x\lt\lt0.075$
Therefore, $x=\sqrt {6.4\times 10^-4\times 0.075}$
$x=[OH^-]=0.022 M$
$pOH=1.66$
$pH=14-pOH=12.34$