The lateral surface area is the area of all the surfaces of the pyramid minus the area of the base.
In this case,
For any rectangular prism, each triangular face is congruent to the opposite face.
[tex]\text{ area of a triangle = }\frac{1}{2}\times\text{ base }\times\text{ perpendicular height}[/tex]
Hence,
[tex]\text{ the lateral surface area = (}\frac{1}{2}\times20\times9)\times2+(\frac{1}{2}\times18\times8)\times2=180+144[/tex]
Therefore,
[tex]\text{ The lateral surface area = 324ft}^2[/tex]
