Nothing in particular. These are not dimensionally equivalent. Length would have a unit such as meters or feet, while width x height would have units such as (square meters) or (square feet), so you cannot just add them.
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The perimeter is 2 times the length plus 2 times the width.
L = 3x + 2y
Capacity = (1/2) x (length) x (width) x ( [height of short end] plus [height of tall end] )
If the width is 9' and the length is twice that, the length is 18'; the perimeter is twice the length plus twice the width, which is 36' + 18' = 54 feet.
Perimeter: Add the length of the four sides. Area: Multiply length x width.