Using Coulomb's Law
F = {k(Q1)(Q2)}/r²
Where F =force in Newtons, k = Coulomb constant, Q1 = - Q2=
charge of the objects in Coulumbs, and r is the distance between the objects.
Solving for r:
r = sqrt[{k(Q1)(Q2)}/F]
where k = 8.897 x 10^9 N•m²/C², Q1 = 2.5 C, Q2 = -2.5 C, and
F = -700 N
Note: Negative magnitude of Force indicates attraction.
r = 8,912.77 m