392 (linear measurement of all four edges) X 2.25 (width of binding) = 882 square inches (the square root tells you that you need about a 30" sq of fabric or similar - 42" X 21" )
I add some extra square inches to the measurement of all four sides to allow for the seam allowance to sew your trapezoid together and for a bit of fudge factor. Rather than 392, I would start with 440. 440 X 2.25 = 990 (square root: 32 square inches)