Same current I is flowing in 3 infinitely long wire along positive x,y and z direction, the magnetic field at a point (0,0,-a) would be

Let the magnetic feild at a point for infinite wire beB.
Then, Here in this case contribution will come from wire alongxandyaxis only.
Which are perpendicular to each other.
So apply ampere circutal law and you can find direction and magnitude ofB.
Let both are flowing along positive direction.
Then, magnetic field at(−a)forxwould be
Bx​​=2πaμ0​I​j^​
And forywould be
By​​=2πaμ0​I​−i^
So the net magnetic field would be
B=2πaμ0​I​(j^​−i^)