Hi



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 

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