Finding Electric Field at a Given Position due to Point Charges

[IsmailKemal]

Homework Statement

Calculate the electric field at a given position due to two point charges +q and -q located on the z axis with a separation d.
q=10^-8 c
P(1 1 1)
d=0.1m

Homework Equations

E=1/4*pi*Eo*r^3*[(3*r'*p'*r'/r^2)-p']
p'=q*d' (d<<r)

The Attempt at a Solution

I think +q charge on the positive z axis and -q charge on the negative z axis so r'=p-(0 0 0). Is that wrong ?
The charges on the z axis with separation d so happy mean d/2 on the z direction. Can ı write r'=p-(0 0 d/2) ?
Which one ı choose ?

 

[haruspex]

It doesn't matter as long as you state your coordinate system. I would set the origin at the midpoint since it will yield a symmetric form.