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**Charges of −12.0 nC and −21.0 nC are separated by 0.700 m. What is the potential midway between the two charges?**

I used the equation V = kq / r for each charge

and then added both V's. But I got it wrong. (the correct answer should be -847 V)

So...

V1 = (8.9x10^9)(-12 x 10^-9) / 0.7 = -152.6

V2 = (8.9x10^9)(-21 x 10^-9) / 0.7 = -267

V1 + V2 = -152.6 + (-267) = -420

What am i doing wrong?