Given:
Perimeter of the rectangle = 84 cm
Breadth of the rectangle = 8 cm
We know that
Perimeter of a rectangle = 2 (Length + Breadth)
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⇒ 84 = 2 (Length + 8 )
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⇒ 42 = Length + 8
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⇒ Length = 42 - 8 = 34 cm
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∴ Length of the given rectangle is 34 cm.