Homework Help: Two coils mutual force (HELP)

1. The problem statement, all variables and given/known data

In a compact cicrcualr coil "A" (radius R= 25cm),formed by N=150 loops of wire, flows I=10A current.
There's another compact circular coil "B" over "A"(radius r= 1.5 cm, loops n= 30) coaxial with "A" distant X ,in which i= 4A flows .
in the same direction of "I" (look at the image).

Calculate the mutual force between the two coils, and is it repulsive or attractive ?
Calculate X value when the force is Maximum.

http://img179.imageshack.us/img179/7660/bobinacn6.jpg [Broken]

HELP ME I DON'T KNOW HOW TO SOLVE IT.....sorry for the translation English isn't my mother tongue. I already posted it in the advanced phys section but maybe this is the right section...


2. Relevant equations

..all about mutual induction, auto induction, coils, magnetic energy

3. The attempt at a solution

 

help me