Three charged particles are placed at each of three corners of an equilateral triangle whose sides are of length 4.0 cm. Two of the particles have a negative charge: q_1 = -6.0 nC and q_2 = -12.0 nC. The remaining particle has a positive charge, q_3 = 8.0 nC. What is the net electric force acting on particle 3 due to particle 1 and particle 2?