Question

A gallon of stain can cover a square deck with an area 300 ft.² about how long is each side of the deck

Answer 1

The area of a square is the square of the length of any side.

A square with 300 square feet of area has sides
that are (√300) feet long.

( √300 ) feet = 17.32 feet (rounded) or  17-ft  3.85-in  (rounded).