Start by writing down a formula for the electrostatic potential $V(\rr)=V(x,y,z)$ everywhere in space due to a single point charge that is not located at the origin.

You should then find the electrostatic potential everywhere in space for the following two cases:

1. Two charges $+Q$ and $+Q$ are placed on a line at $z=+D$ and $z=-D$, respectively.
2. Two charges $+Q$ and $-Q$ are placed on a line at $z=+D$ and $z=-D$, respectively.

Simplify your expressions, assuming that you are evaluating the potential along the $z$-axis. Then do the same, this time assuming you are on the $x$-axis.