Consider the two parallel wires as shown in the following figure.
Now, force experienced by the two wires is F=
µ0
4π
2I1I2
d
We know I1=1A;I2=3A;d=1m and
µ0
4π
=10−7 Therefore, F=
10−7×2×1×3
1
=6×10−7 and since the currents are in opposite directions, therefore, force experienced will be repulsive So, the force per unit length acting between two wires is 6×10−7Nm−1 and it is repulsive.