Take two identical set squares with angles and place them adjacently to form a parallelogram as shown in Fig 3.20. Does this help you to verify the above property?
Property : The opposite sides of a parallelogram are of equal length.
Property : The opposite sides of a parallelogram are of equal length.
As we can see in the figure above, the opposite sides of figure are equal.
The figure above is a rectangle which is part of parallelogram.
Hence, the opposite sides of a parallelogram are of equal length.