Zafar,
I am having some trouble following your question, as you said graphics would make this much easier.
1) You have a Regular Octagon with base side 30cm, all sides are equal?
2) There is a rectangle in the middle of the Octagon, no sides touching the Octagon?
3) That rectangle has length 42cm and height 21cm?
4) Area shaded is actually area NOT within the rectangle, otherwise this would simply be solve for the area of the rectangle.
Comments
I am having some trouble following your question, as you said graphics would make this much easier.
1) You have a Regular Octagon with base side 30cm, all sides are equal?
2) There is a rectangle in the middle of the Octagon, no sides touching the Octagon?
4) Area shaded is actually area NOT within the rectangle, otherwise this would simply be solve for the area of the rectangle.