Your question says 75 square feet, but the question in the picture says 24 square feet.
For the one in the picture:
Area of a square is S^2 to find the side when you know the area, take the square root of the area.
Side = √24, which is re-written as 2√6 which = 4.89, which rounds to 4.9 ft.
The answer is the last choice.