First note the intersections of each pair of lines.
x = 3 ⇒ y = 2•3 + 1 = 7 ⇒ (3, 7)
y = -3 ⇒ -3 = 2x + 1 ⇒ x = -2 ⇒ (-2, -3)
y = -3 and x = 3 ⇒ (3, -3)
Using the disk method, we consider disks with thickness ∆y and radius equal to the horizontal distance between the line y = 2x + 1 (or x = (y - 1)/2) and the axis of revolution, x = 3. Each disk will then contribute a volume of
∆V = π (radius)² (thickness) = π/4 (y - 1)² ∆y
As we let ∆y go to zero and let the number of disks go to infinity, the total volume of the resulting cone will be given by the integral
