Find the volume of a rectangular prism if the length is 4x, the width is 2x, and the height is x^3 + 3x + 6. Use the formula V = l * w * h, where l is length, w is width, and h is height, to find the volume.
6x^5 + 18x^3 + 36x^2
6x^6 + 18x^3 + 36x^2
8x^5 + 24x^3 + 48x^2
8x^6 + 24x^3 + 48x^2