Question 435681
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Hi
<img src="http://upload.wikimedia.org/wikipedia/commons/thumb/d/da/PerimeterRectangle.svg/200px-PerimeterRectangle.svg.png">
length is twice as long as the width
Let x and 2x represent the width and length respectively
 2*2x + 2x = 21cm
    6x = 21cm
     x = 3.5cm, the width.  the length is 7cm  (2*3.5cm)

CHECKING our Answer***
  2*7cm + 2*3.5cm = 21cm