Currents I1 and I2 flow in the wires shown in figure. The field is zero at distance x to the right of O. Then
x=I1I2a
x=I2I1a
x=I1−I2I1+I2a
x=I1+I2I1−I2a
Magnetic field due to both wires will be equal and opposite in diretion μ0I14π(a+x)=μ0I24π(a−x) ⇔a−xa+x=I2I1 ⇒x=I1−I2I1+I2a.