Because the base of the notepad is a square, that means that each side of the base would be equal. Since the area is 2.25 square inches, and each side would have to be equal, the equation to solve would be x*x = 2.25, or x^2=2.25. So, you just take the square root of both sides, and x=1.5 inches. Each side of the notepad would be 1.5 inches.
