Since, the wires are infinite, so the system of these two wires can be considered as closed rectangle of infinite length and breadth equal to d. Flux through strip ϕ=
d−a
∫
a
µ0I
2πr
(ldr)=
µ0Il
2π
ln(
d−a
a
)
The other wire produces the same result, so the total flux through the rectangle ABCD is ϕtotal=