After brushing, fluffy's fur has a charge of +8.0 × 10-9 coulombs and her plastic brush has a charge of –1.4 × 10-8 coulombs. if the distance between the fur and brush is roughly 5.0 × 10-1 meters, what is the approximate magnitude of the force between them? (k = 9.0 × 109 newton·meters2/coulomb2)

The equation in Coulomb's law is F ={k*q1*q2}/r^2 where F is the force between charges, k is a constant, q1 is the charge of the first particle, q2 is the charge of the second particle and r is the distance between the two particle charges. 

Substituting to the equation:
F =k*q1*q2/r^2 = { 9x10^{9} * 8x 10^{-9} *-1.4x10^{-8} }/ ((5x10^-1)^2)
= -4.032x10^-6 Newtons