The electric field at a point $(x, 0, 0)$ on the x-axis due to the charge +q is given by Coulomb's law:
$\vec{E}_+ = \frac{1}{4\pi\epsilon_0} \frac{q}{x^2 + (d/2)^2} \hat{r}_+$
where $\hat{r}_+$ is the unit vector pointing from $(0, 0, d/2)$ to $(x, 0, 0)$.
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