We want to determine the total surface area of the tent. The surface area would be
Area of 2 triangular faces + area of the opposite rectangular faces + area of the rectangular base
Since the triangular face is an isosceles triangle, we it means that the two opposite sides are equal. Thus, the side lengths are 26ft, 26ft and 20ft. We would find the area of the triangular face by applying heron's formula which is expressed as
Area of 2 triangular faces = 19.596 x 2 = 39.192
Area of 2 opposite rectangular faces = 2 x 26 x 40 = 2080
Area of rectangular base = 20 x 40 = 800
Total Surface area = 39.192 + 2080 + 800 = 2919.192
Thus, the minimum number of square feet needed is 2919.192 square feet