Two charges, - q0 and - 4q0 are a distance l apart. these two charges are free to move but do not because there is a third charge nearby.

Unfortunately, it was not stated in the problem above what is being asked of us to perform in this problem but just for the general note, the magnetic field between charged particles can be calculated through the Coulomb's law.

F = kq₁q₂/r²

where k is the Coulomb's constant, q₁ and q₂ are the charges, and r is the distance between them.