Two point charges of the same sign, $q = 4$ uC each, repel each other with a force $F = 9$ N. Find the distance between the charges. Coulomb's constant is $k = 9 \cdot 10^9$ N·m²/C².
From Coulomb's law $F = kq_1q_2/r^2$ express the distance:
$$r = \sqrt{\frac{k q^2}{F}} = \sqrt{\frac{9 \cdot 10^9 \cdot (4 \cdot 10^{-6})^2}{9}} = \sqrt{1.6 \cdot 10^{-2}} \approx 0.126 \text{ m}$$
$r \approx 0.126$ m (12.6 cm)