In a particular region there is a uniform current density of 15
$\mathrm{A} / \mathrm{m}^{2}$ in the positive $z$ direction. What is the value of $\ \vec{B} \cdot d \vec{s}$ when that line integral is calculated
along a closed path consisting of the three straight-line segments from $(x, y, z)$ coordinates $(4 d, 0,0)$ to
$(4 d, 3 d, 0)$ to $(0,0,0)$ to $(4 d, 0,0),$ where $d=$
20 $\mathrm{cm} ?$