Deriving V-E Relation for Uniformly Charged Rod at Radial Distance r > a"

[SpatialVacancy]

Could someone help me out on this problem?

Derive a relation between the potential $V$ and the magnitude of the Field $E$ at a radial distance $r$ from the axis
of a very long uniformly charged rod of radius $a$ ($r > a$).

 

[Nylex]

I'm not following that properly. At first I would have thought you use Gauss' Law for electrostatics to work out the E field at a point r from the rod and then use E = -dV/dr, but that doesn't seem right :/.

 

[Tide]

I'm not sure what you mean by "derive a relation between..." since $\vec E = -\nabla \Phi$ is fundamental.