To find area you have to do length times width, and for squares, you just multiply the length or width by itself since a square's length and width are the same. Since you know that the side length of the square is 5√9, you multiply 5√9 by 5√9. First you multiply 5 by 5 which is 25, then you multiply √9 by √9 which is 9. Then you multiply 9*25 and you get that the area of the square is 225.