Answer:

The side of the square is the hypotenuse of a right triangle.  The Pythagorean Theorem can be used to find the length of the side of the square.  Since the square has four equal sides, this value can be multiplied by 4 to find the perimeter.

The length of the hypotenuse is 5 ft so the perimeter would be 20 feet.

3^{2   +}  4²   =  5²