For a given perimeter, the area will be maximized when all the sides are the same length, which makes it actually a square. A square is still a rectangle, though! So, if you know the perimeter, divide it by four to determine the length of each side. Then multiply the length times the width to get the area.
btw Hannah is this one fine? sorry i couldn't remember which question was the chat and sorry i couldn't respond right away im not the person who is on late in the day