So db =1/2ab =1/2 * 7 = 3.5;

BE = 1/2BC = 1/2 * 6 = 3；

EF = 1/2BA = 1/2 * 7 = 3.5；

FD= 1/2CB= 1/2*6=3

That is, the perimeter of the quadrilateral dbef = db+be+ef+FD = 3.5+3+3.5+3 =13.