the perimeter is the sum of the length of every side of the geometrical figure. In your case, since you have 4 sides two of which are equal, the perimeter would be 2*(a+b) where a and b are the length of two sides
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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.
I believe you've already found it.
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