$1)$ The current density, J$,$ inside a long, solid, cylindrical wire of radius a $=4\mathrm{.}1$ mm is in the direction of the central axis.$$

a$)$ If its magnitude is a constant Jo $=210$A$/$m$2,$ find the magnitude of the magnetic field at r $=3\mathrm{.}00$mm

If its magnitude varies with radial distance r from the axis according to J $=$ Jor$2/$a$2,$ where Jo $=210$ A$/$m$2,$ find the magnitude of the magnetic field at r $=3\mathrm{.}00$mm