This force is called an electrostatic force. To solve for the value of the electrostatic force. Using Coulumbs Law:
F = k * abs (Q1 * Q2) / r^2
where:
k = coulumb's constant, 8.988x10^9 N *m^2/ C^2
Q1 = first charge
Q2 = second charge
r = distance between charges
8.8x10^4 = 8.988x10^9 * abs((-7.5x10^-9) * (11x10^-9)) / r^2
solve for r