Parallelograms have opposite equal sides, then we only need to espcify two sides of the figure, the horizontal side and the diagonal one.
The perimeter of a parallelogram is given by the following formula:
Where a is the length of the horizontal sides (AD and BC) and b is the length of the diagonal sides (AB and CD)
We are told that the side AB is 5 in less than BC, this means that the length of AB equals the length of BC minus 5, then we can formulate the following expression:
AB = BC - 5
As mentioned above, we can replace "b" for AB and "a" for BC