Two particles are fixed to an x axis: particle 1 of charge -5.15 x 10-7 C is at the origin and particle 2 of c?

Two particles are fixed to an x axis: particle 1 of charge -5.15 x 10-7 C is at the origin and particle 2 of charge +5.15 x 10-7 C is at x = 21.0 cm. Midway between the particles, what is the magnitude of the net electric field?