The greatest area bounded by four straight edges occurs when those edges form a cyclic quadrilateral (all four vertices lie on a circle). Given that we can choose the length of wall that we use, it makes send for that to be the diameter of the circle, with length 2r. Then the three fences each form isosceles triangles with r as the common lengths and 25, 33 and 39 as the ‘bases’. It turns out that for this to happen, r=32.5, and the three triangles have ‘heights’ of 30, 28 and 26 respectively. The area is therefore 1344 square metres.