Step-by-step explanation:
E1 = k Q1 / (0.075 m)^2
= 9 x 10^9 Nm^2/C^2 * 10 x 10^-9 C / 0.075^2 m^2
= 400 N/C
E2 = k Q2 / (0.075 m)^2
= 9 x 10^9 Nm^2/C^2 * (-3 x 10^-9 C) / 0.075^2 m^2
= -120 N/C
Since the electric fields due to the two charges are in opposite directions, we can find the net electric field at the midway point by taking the difference between the two:
E = E1 - E2
= 400 N/C - (-120 N/C)
= 520 N/C