Question 1164005
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Perimeter = 2L + 2W

Length L = 2W.


So, replace L by 2W in the first equation, based on the second relation.  You will get

    2*(2W) + 2W = 10.5

    4W     + 2W = 10.5 

    6W          = 10.5

     W          = 10.5/6 = 1.75.


Thus the width of the cross-section is 1.75 in.


Then the length of the cross section is  2*1.75 = 3.5 in.
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Solved.