Derivation Problem (Magnetic Field)

[Solisiel]

Homework Statement

Hi, I'm having trouble solving this problem:

I can't seem to get to the right derived formula. The one selected is wrong. I'm still attempting to solve this.

Homework Equations

B= 8 M₀ N (I - I₀)
125^1/2 a

e = 2V
m B² R²

The Attempt at a Solution

Here's my attempt at it.

 

[andrevdh]

Seems to me that it is the 6th answer from the top since
B_f = 2B_j where B_j = φ(I_j - I_o)