The force is given by this equation
F = k *q1*q2
------------
r^2
where K is Coulomb's constant = 9.10^ 9 [N.m^2 /C^2]
q1 is the value of charge 1, q2 the value of charge 2
r is the distance between them
F = 9*10^9 * -21.10^-6 * 47.5 10^-6
---------------------------------------------- = -15716,636 N (Negative so it is Attractive Force)
(0.0239)^2