Math Help - wire potential

1. wire potential

Hi ,
I would like some help with the following problem:

A high-voltage power line(cable radius a=2cm) is at a voltage V=345[kV]( with respect to ground). The power line is at a height h=50[m] above the ground which may be approximately modeled as an infinite ground plane( see picture attacjed).
1-Calculate the electric field vector at a point on the ground directly below the line. Calculate field vector at the point on the power line that is closer to the earth (the bottom of the line).
2-Estimate the voltage at wich dielectric breakdown happen(Ec=3[MV] for the given a.
3-Estimate the radius radius of the power line at which dielectric breakdown happens for the given V

Assume that I have found the first question.

For the second question: I have done a problem where I needed to compute the voltage between 2 wires of radius a and distant center to center by the distance h.
I would like to know if I can use this result to answer the question 2 and 3.
I have some concern about the fact that there is the ground plane.( There is some restriction on how far should y varies because of the groun plane)
It was V=[rho* ln(a/h)]/(pi*epsilon)[V]

If I cannot use this result , please what can I do?
thank you
B