In a particular region there is a uniform current density of 18 a/m2 in the positive z direction. what is the value of line integralvector b·dvector s when that line integral is calculated along 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 = 30 cm?