Answer:
 9 cm
Step-by-step explanation:
You want the side length of a square with the same perimeter as a rectangle that is 8 cm by 10 cm.
Side length
The side length of the square will be the average of the side lengths of the rectangle: (8 cm +10 cm)/2 = 9 cm
The length of a side of the square is 9 cm.
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Additional comment
The perimeters of the two figures are equal, so you have ...
 2(L+W) = 4S
 (L+W)/2 = S . . . . . divide by 4; the average of length and width
