Electric potential midway between 2 charges

[kbyws37]

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?

 

[denverdoc]

maybe just algebra, the distance you need to use is the 1/2 way point of 0.35 see if that helps.