The force on the -2.5x10-6 C charge is given by Coulomb's Law:
F = k*(q1*q2)/r^2
where k is the Coulomb's constant (8.99x10^9 Nm^2/C^2), q1 and q2 are the charges, and r is the distance between them.
F = 8.99x10^9 Nm^2/C^2 * (-2.5x10^-6 C) * (3.0x10^-6 C) / (0.50 m)^2
F = -7.49x10^-5 N
The force on the 3.0x10-6 C charge is the same magnitude but in the opposite direction:
F = 7.49x10^-5 N