"two long parallel wires that are 0.30 m apart carry currents of 5.0 a and 8.0 a in the opposite direction. find the magnitude of the force per unit length that each wire exerts on the other wire and indicate if the force is attractive or repulsive. (μ0 = 4π × 10-7 t • m/a) "

The magnetic field B generated by wire 1 at distance r (where wire 2 is located) is

where I1 is the current in wire 1. So, the force exerted on a segment  of wire 2 is

By susbstituting B inside the expression of F, we find the formula for the force per unit of length between the two wires:

where r is the distance between the two wires.
By susbstituting the numbers of the problem, we get