The current density in a cylindrical wire of radius r = 4 . 0mm is...

Current $= J \times$ Area

$= J \times \pi{\left[{R}^{2} - \left(\dfrac{R}{2}\right)\right]}^{2} = J\pi\dfrac{3{R}^{2}}{4} = {10}^{6} \times \pi \times 3 \times {\left(\dfrac{4 \times {10}^{- 3}}{4}\right)}^{2}$

$⇒ \pi \times 3 \times \dfrac{16}{4} = 12\piA$

Hence, the value of $x = 12$.