First, you get the area of the triangle like this in the attachment and you get 108 and you times 15 and 20 to get 300 add those up to "fill" up your 3D shape
Well, you do the area of each individual side and add them together. The triangles have an area 108 square inches. The two 15 inch sides each have an area of 300 square inches each. And the bottom face has an area of 480 inches squared. You add them all together to get= 108+108+300+300+480, which is equal to 1,296 square inches.