Answer
(a) negative y axis
(b) $43.0cm$
Work Step by Step
(a) We know that according to the right hand rule, the magnetic field due to the long straight wire points in the positive z-direction along the positive y-axis and points in the negative z direction along the negative y-axis.
(b) We know that
$B_{uniform}=\frac{\mu_{\circ}I}{2\pi r}$
This can be rearranged as:
$r=\frac{4\pi \times 10^{-7}\times 3.12A}{2\pi(1.45\times 10^{-6}T)}$
$r=43.0cm$
