the area of a square is 361 sq ft. Find the length of one side of the square

The area of a square is its length times its width:  l x w  =  361

Since the length of a square is the same as its width, replace 'w' with 'l':  l x l  =  361

     l x l  =  l²  =  361     --->   l  =  √361       --->   l  =  √361       --->     l  =  19 ft.