we have that
If you draw a diagram, and draw perpendiculars from the intersection to the sides of the rectangle, you will see they are the mid point of each side.
see the attached figure
If the mid point of the smaller side is ---------> x.
The mid point of the larger side will be -----------> (x + 4)
The full length if the smaller side will be 2x and the length of the larger side will be 2(x + 4) = 2x + 8
The perimeter = 2 times the length + 2 times the width.
2(2x + 8) + 4x = 56
4x + 16 + 4x = 56
8x = 40
x = 5
Therefore
length = 2x + 8 = 18cm
width = 2x = 10cm
the answer is
the lengths of the sides of the rectangle are
length = 18cm
width = 10cm