Halliday, Resnick & Walker — Magnetic Fields Due to Currents: Problem 38

38 In a particular region there is a uniform current density of \(18 \text{ A/m}^2\) in the positive \(z\) direction. What is the value of \(\oint \vec{B} \cdot d\vec{s}\) when that line integral is calculated along a closed path consisting of the three straight-line segments from \((x, y, z)\) coordinates \((4d, 0, 0)\) to \((4d, 3d, 0)\) to \((0, 0, 0)\) to \((4d, 0, 0)\), where \(d = 20 \text{ cm}\)?