Length + width = 18 cm so length = 12 cm and width = 6 cm. Presumably the length is the common side so the perimeter of the octagon is 8 x 12 = 96 cm. If the width is the common side then the perimeter of the octagon is 8 x 6 = 48 cm
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The perimeter of a rectanble is the length of all it's sides. So it depends on the size of the rectangle, it could be millimetres or miles long or wide.
In general, an octagonal pyramid is a pyramid with an octagon for a base and triangles (which all have a common vertex) for sides. The octagon my or may not be a regular one.
Area = length x width you may try the online calculator linked below that is related to this area and perimeter calculation of common shapes like rectangle, circle, triangle, parallelogram, rhombus and trapezoid.
equal sides
One thing that area and perimeter has in common is that they both measures some part of a shape