Answer
$1.37m$
Work Step by Step
We can find the required separation as
$F=K\frac{q_1q_2}{r^2}$
This simplifies to:
$r=\sqrt{\frac{Kq_1q_2}{F}}$
We plug in the known values to obtain:
$r=\sqrt{\frac{(8.99\times 10^9)(11.2\times 10^{-6})(29.1\times 10^{-6})}{1.57}}$
$r=1.37m$
