Check the picture below.
so each triangular face has a height of 91.4 and a base of 115, and there are four of them, now, we're only considering the "lateral area", that excludes the base.
[tex]\stackrel{\textit{area of 4 triangles}}{4\left[ \cfrac{1}{2}(115)(91.4) \right]}~~\implies ~~ 21022[/tex]
