check the picture below.
since the slant height of the pyramid is 29, that'd be the altitude or height of a triangular face, whose base is 40.
we can simply get the area of the the base, it's just a 40x40, and the area of all 4 triangular faces and sum them up, and that's the surface area of the pyramid.
[tex]\bf \stackrel{\textit{area of the base}}{(40\cdot 40)}~~~~+~~~~\stackrel{\textit{area of the 4 triangles}}{4\left[ \cfrac{1}{2}(40)(29) \right]}[/tex]
