the area of the shaded reigon=octagonarea-squarearea
octogon=8 times 1/2 times base times apothem or
4 times base times apothem=4 times 15 times 18.11=1086.6 square units
the area of the rectangle (or square) is side times side or 15*15=225
the shaded area=1086.6-225=861.6 square units
the shaded area is 861.6 square units
