(a) For a uniform field $\vec E=E_0\hat i$, equipotential surfaces are planes perpendicular to $\hat i$ (i.e. parallel to the y-z plane). Draw three such parallel, equally-spaced vertical planes/lines at increasing $x$, with potential decreasing in the direction of $E$.

(b) Dipole moment $p=q\times2a$. For an axial point at distance $x$ from the centre with $x\gg a$:

$$V=\frac{1}{4\pi\varepsilon_0}\frac{p}{x^2}=\frac{1}{4\pi\varepsilon_0}\frac{2qa}{x^2}$$