Answer:
F = 3.105 10⁻²⁰ N
Step-by-step explanation:
For this exercise let's use Coulomb's law
F = k q₁ q₂ / r²
in this case each of the grains has a charge
q1 = 6.0 10⁻¹⁰ C
q2 = 2.3 10⁻¹⁵ C
and they are separated by a distance r = 2 cm = 0.02 m
let's substitute
F = 9 10⁹ 6.0 10⁻¹⁰ 2.3 10⁻¹⁵ / 0.02²
F = 3.105 10⁻²⁰ N