In order to find the perimeter of a 3D rectangle you must gather the lengths of the known sides, calculate the missing rectangular values, and use the formula for perimeter.
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Height = (Perimeter/2) - Base
The perimeter of a rectangle = 2 x (length + width) = 2 x (12 + 8) = 2 x 20 = 40 inches
Length = (Perimeter - twice width) / 2
The perimeter of a rectangle is not sufficient to determine its length.