Help with pre-cal problem?

I need help with this word problem: The number of square feet in the area of a square banquet room is 357 greater then the number of feet in its perimeter. Determine the dimensions of the room. I know the answer is 21x21, but the teacher expects us to work it out. How do you do it? I'm taking a wild guess here, but do you divide the 357 into 4? She's really a bad teacher, and she's too stubborn and lazy to actually show us the work. Can anyone help me?