Question

In the figure below, a long circular pipe with outside radius R = 2.57 cm carries a (uniformly distributed) current i = 10.4 mA into the page. A wire runs parallel to the pipe at a distance of 3.00R from center to center. Find the magnitude of the current in the wire in milliamperes such that the ratio of the magnitude of the net magnetic field at point P to the magnitude of the net magnetic field at the center of the pipe is 4.74, but it has the opposite direction.
http://i48.tinypic.com/hstmop.jpg
When I did it I got -3.9 mA

Answer 1

its simply based on lenz's law

Answer 2

It's about Ampere's Law.