A long circular pipe with inside radius a and outside radius b carries a current out of the page. Current density in the pipe is given by $J=J_{0}r.$ A wire runs parallel to the pipe at a distance of $x$ from the center of the pipe $($see Fig$\mathrm{.}2).$ The wire carries a current I into the page.

a$)$ Find the magnitude $B$ of a magnetic field due to the pipe at a distance $r$ from the center of the pipe if $a.$

$b$