Magnetic field of 2 infinite wires.

Homework Statement

Two thin, long wires situated in y = -a and y = a carrying two equal in magnitude and opposite running currents I.
Find the field strength in therms of y.
Find the average Field strength in a region -b to b, where 0 is between the 2 points.

Amperes law.

The Attempt at a Solution

I find via. Amperes' law, that the field of the wire in -a (current running down) has the magnitude:
$B(y) = \dfrac{\mu_0 I}{2\pi r_1}$
Where r1 is the distance from the wire, now i find that r1 = a-y.
The field from the other is the same with the distance r2 instead of r1. r2 = a-y too so the field is zero and the average is of course also 0, and that is annoying. What have i done wrong.

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