Question 1006936
If you are looking for the shaded area, you need not be concerned about the perimeters.  You subtract the area of the small square from the area of the large square, or
A(shaded) = (x+5)^2 - (x-8)^2 = x^2 + 10x + 25 - (x^2 - 16x + 64) =
A(shaded) = 26x - 39 inches^2