Question 508891
Let 'x' be the unknown measurement.
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Perimeter of a triangle = sum of sides.
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3x + 5x + 6x = 21
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14x = 21
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x = 21/14 = 3/2
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3x = 3(3/2) = 9/2
5x = 5(3/2) = 15/2
6x = 6(3/2) = 18/2
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9/2 + 15/2 + 18/2 = 42/2 = 21
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The three sides are:  9/2, 15/2 and 18/2
or 4.5, 7.5, and 9.
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Done.