Question

Two particles are fixed to an x axis: particle 1 of charge -1.00 x 10-7 C is at the origin and particle 2 of charge +1.00 x 10-7 C is at x = 17.1 cm. Midway between the particles, what is the magnitude of the net electric field?

Answer 1

sorry sorry its sum of two fields due to two charges

Answer 2

Since: \[\vec{E}=k_{e}\sum_{i}\frac{q_{i}}{r^2}\hat{r_{i}}\]
you simply add up the electric fields from the two point-charges. P1 will be toward the origin, as will P2 (remember E is considered for a positive test charge).