Perimeter of a parallelogram is the total length of its sides and it is calculated by adding together the length of all its sides (as it is calculated for any other polygon).
P = a + a + b + b
Where P is perimeter of the parallelogram, a is the longer side, and b is the shorter side.
If a = 8 units and b = 5 units then the perimeter of the parallelogram is:
P = a + a + b + b = 8 + 8 + 5 + 5 = 26 units