Abcd and decf are both squares. if ac = 28 millimeters, what is the perimeter of decf?

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

They must get seven yeards.