In quadrilateral ABCD, side DC is 4, side AD is 7, side AB is equal to the difference

In quadrilateral ABCD, side DC is 4, side AD is 7, side AB is equal to the difference between sides AD and BC. Find the length of the side DC if the perimeter of the quadrilateral ABCD is 27.

We take the side BC for x, then AB = AD-BC = x-7 by the problem statement.
Perimeter P = AB + BC + CD + AD by definition, then we get the equation:
x-7 + 4 + 7 + x = 27
2x = 27-4 = 23
x = 11.5

AB = 11.5-7 = 4.5
Checking: 4 + 7 + 11.5 + 4.5 = 27 The result matches the one specified in the condition.