Well, your "reasoning" is wrong. The volume is the same but the distance the gas has to be "lifted" is different. With the tank on its side, every horizontal "slice" of gas is a rectangle with length equal to the length of the tank, 4 feet, and width equal to the length of a chord of the circle. If we set up a coordinate system with center at the center of circle, its equation is . The width of the circle at a given y is . Taking "dy" as the thicknes of a "slice", its volume is . Multiply that by the density of the gas and the height that "slice" has to lifted, 8- y (the top of the tank is at y= 3 and it has to be lifted another 5 feet), and integrate from y= -3 to y= 3.