If the formula for the perimeter of a rectangle is P = 21 + 2w, which is an expression for w?
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If the formula for the perimeter of a rectangle is P 2l 2w
As you stated the formula of a rectangle is P (perimeter) = 2W (the two widths added together) + 2L (the two lengths added together). To solve this algebraicly you need to define your variable, or unknown, which in this case is the width or "W"!
hope this helps!