So electric field equation is E=kq/r^2. There are two charges, and it is looking for the electric field midway. So midway also means half the distance from the two charges, so 8.4 cm/2=4.2 cm (or 4.2*10^-2 meters). Find the e field of the two individual charges and add them up to have a net e field.
E-field 1) E=8.99*10^9(-7*10^-6)/((4.2*10^-2)^2) = -3.57*10^7 N/C
E-field 2) E=8.99*10^9(7.2*10^-6)/((4.2*10^-2)^2) = 3.67*10^7 N/C
Total E-field = -3.57*10^7+3.67*10^7 = 1.02*10^6 N/C