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 Hi Syed, I reproduced the diagram you sent and added the letters $A, B, C, \mbox{ and }D.$ 
 Is this all you know? The perimeter is $8 + 4 + |CD| + |DA|$ and since triangle $ABC$ is a right triangle you can use Pythagoras theorem to show that $|CA| = \sqrt{8^2 + 4^2} = \sqrt{80}.$ From this information you can't determine $|CD|$ or $|DA|,$ all that you can say is that $|CD|^2 + |DA|^2 = 80^2.$ Penny  | ||||||||||||
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