Add the length of all nine sides up.
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It is the sum of the lengths of the nine sides of the nonagon.
21*9 = 189 feet.
If you know the perimeter, there is no need to find it again.
There is no reason for the perimeter of a triangle to have any relation to the perimeter of an unrelated rectangle!
There are many formulas for perimeter depending on what shape you are trying to find the perimeter of. The perimeter is the distance around a shape, so one formula to find perimeter is simply adding all the side lengths together.