Используем формулу разности двух квадратов:
a²-b² = (a-b)(a+b)
(x+3y)²-(3x-у)² = (x+3y-3x+y)(x+3y+3x-y) = (4y-2x)(4x+2y) = (2(2y-x))(2(2x+y)) = 4(2y-x)(2x+y);
(5x+y)²-(x-5y)² = (y+5x)²-(x-5y)² =(y+5x-x+5y)(y+5x+x-5y)=(6y+4x)(6x-4y) = (2(3y+2x))(2(3x-2y)) = 4(3y+2x)(3x-2y).