Let x be the smaller sheet.
Smaller sheet = x
Larger sheet = x + 3
Area of the smaller sheet = x²
Area of the larger sheet = (x + 3)²
The difference is 81 in²
(x + 3)² - x² = 81
x² + 6x + 9 - x² = 81
6x + 9 = 81
6x = 72
x = 12 inches
Answer: The edge of the smaller square is 12 inches.