AB=a; BC=b+b=2b; BC=EF=AD=2b; EC=x+x+x=3x; EC=FB=3x; AF=AB−EC=a−3x; AF=DE=a−3x.
Perimetro del rettangolo AFED=(AF+DE)+(AD+FE)=(a−3x+a−3x)+(2b+2b) = (2a−6x)+(4b) = 2a−6x+4b=2(a−3x+2b).
Area del rettangolo AFED=AF⋅AD=(a−3x)⋅2b=2ab−6bx.