Answer:
AB = 3.3 cm
Step-by-step explanation:
The formula to find out area of a regular octagon is given by

where a is the length of each side of the regular octagon.
Plugin A=54 into the formula

Divide both sides by 2


Plugin √2 as 1.41


Divide both sides by 2.41


Taking square root on both sides

a = 3.346
a = 3.3 cm (rounded to nearest tenth)
so, length of side AB = 3.3 cm