A "pulls" up on B, while C "pulls" down. So the net force, taking up as positive, is
F = ΣkQq / d² = 8.99e9N·m²/C² * 2.0e-9C * (1.0e-9C / (0.021m)² - 2.0e-9C / (0.013m)²)
or
F = -1.7e-4N
(that is, "down")
Havent done physic 12 for a while! Your teacher should have given u some formulas.i believe its the one with a c1c2 or a p1 p2. Send me that and i can get it