ax - ay + bx - by= (ax - ay) + (bx - by)
= a(x - y) + b(x - y)
= (x - y)(a + b)
x2 - 2xy + y2 - 1= (x2 - 2xy + y2) - 12
= (x - y)2 - 12
= (x - y - 1)(x - y + 1)
9 - x2 - 2xy - y2 = 32 - ( x2 + 2xy + y2 )
= 32 - ( x + y)2
= ( 3 - ( x + y)).(3 + ( x + y))
= (3 - x - y)(3 + x + y)