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