Net magnetic field due to two wires

[carpelumen]

Homework Statement

The two wires shown in the figure carry currents of I = 5.58 A in opposite directions and are separated by a distance of d0 = 8.43 cm.
Calculate the net magnetic field at a point midway between the wires. Use the direction out of the page as the positive direction and into the page as the negative direction in your answer.

http://tinypic.com/r/258ufyq/8

Homework Equations

2*u0*i / 2*pi*r

The Attempt at a Solution

2(4piE-7)(5.58) / 2(pi)(.04215) = 5.286E-5 THw system says it's incorrect, I don't know where to go from here :(

 

[NascentOxygen]

Step 1 is to draw a diagram, and mark on it relevant information. Identify the point of interest.
Step 2 is to decide whether the fields will [tend to] add or cancel at the point of interest. (Should they exactly cancel then there is no further calculation to be done!)
Step 3 is to finish working it out. How will you approach this?