39. Given that J- 15 f (3x) dx = - 15, which of the following must be true? A. 1 45 35 - 45f (r) dr = - 5 B. 1 45 3) _ Asf (r) dr = - 15 C. 3/_ f (r) dr = - 5 D. - 3/15 15 f (r) dr = - 15 E. - 3" (f (r) dr = - 45 40. The base of a solid is the region in the first quadrant bounded by y = xandy = x - 6. If the cross-sections perpendicular to the y - axis are squares, what is the volume of the solid? A. 6.069 B. 3.768 C. 7.402 D. 9.003 E. 7.114