Same current i is flowing in three infinitely long wires along positive x-, y- and z-directions. The magnetic field at a point (0, 0, -a) would be

Point (0, 0, -a) lies on z-axis, therefore magnetic field due to current along z-axis is zero and due to rest two wires is μ02πia in mutually perpendicular directions along positive y direction and negative x direction.∴  B→=μ02πia(j^-i^)