Two electrostatic point charges of +67.0 µC and +50.0 µC exert a repulsive force on each other of 172 N. What is the distance between the two charges? The value of the Coulomb constant is 8.98755 × 109 N · m^2/C^2.
F = k*q1*q2/r^2
so plug in your numbers and solve for r.