Electric Charges HELP!

[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.