Question

What is the magnetic flux density midway between two long parallel wires separated by 10 cm each carrying a current of 5A.

a - In the same direction

b- In opposite direction

What is the force per unit length exerted by each wire on the other?

Answer 1

You now need \[B = \frac{\mu_{o} I}{2 \pi r} \]
where r is the distance from the wire.

Answer 2

For the last part you can look at this:
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/wirfor.html#c1
(this website is great for looking things up, though the correct page is usually easiest to find using the method of searching google for "#what you want# hyperphysics"