Perimeter of a rectangle is 2l + 2w. Area is a different formula. Length 7 and width 3 give perimeter 20, not 21. Area of the same shape is in area of a rectangle.
Walk the edge, do not fill it
Fence, frame, or border wants 2l + 2w. Floor or paint wants length times width. Area and volume as a set are in area and volume.
The short list is in ACT Math formulas.
The bottom line
Perimeter: 2l + 2w. Area: lw. Do not swap them.
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