Respuesta :

the complete question in the attached figure

we know that

in the square ABCD
diagonal AC is equal to diagonal BD
EC=AC/2--------> EC=28/2-----> 14 mm

EC is the length side of the square DECF 
so
perimeter of the square DECF=4*EC------> 4*14-----> 56 mm

the answer is
56 mm
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