Electric field question.

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Hum. Confused.

Two charges -4.0mC and -5.0mC are separated by a distance of 20cm. What is the electric field halfway between the charges?

Coulombe's law only tells me the force on a particle next to another one. I don't know the formula for calculating an electric field, let alone what it equals halfway between two charged particles. any help is appreciated.
Look guys, this isnt homework and im not asking anyone to do it for me. Its just in the review for a huge test (the final) tomorrow and i cant remember back to when we studied it. If someone could even just give me the equation so i could solve it myself id be most grateful.
Electric field = Force / Charge
The electric field at distance r, in this case 0.1 m, from charge Q is:

Q / (4pi*E0*r^2)

where E0 is the permitivity of free space. In your case, you will need to find this for both values of Q and add them together.

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