(a) the rectangle [0, 3] x [4,6].

(b) the rectangle [0,3] × [0,3].

(c) the region bounded by the line y = 3x +2 and the x-axis, for x = [0,3].

(d) the region bounded by the line y = 3x + 2 and the x-axis, for x = [0, ∞0].