Final answer:
The expression that represents the length of one of the playground's sides is (56 + 8/3 x).
Step-by-step explanation:
The perimeter of a square is found by multiplying the length of one side by 4. In this case, the expression (56 + 8 left(13xright)) represents the perimeter of the playground. To find the length of one side, we need to divide the total perimeter by 4. Therefore, the expression that represents the length of one of the playground's sides is (56 + 8/3 x).