The length of the side of the square is 6.81 correct to 3 significant places, that implies we can write the length as 6.810 cm.
The perimeter of the square is 6.810*4 = 27.240 cm. As the length of the side is known to an accuracy of 3 significant places, the perimeter too is accurate to 3 significant places.
The lower bound of the perimeter is the number with one extra significant figure that when rounded up would yield 27.240. Here it is 27.2350 cm.
The lower bound of the perimeter is 27.2350 cm.