a) x2 - y2 + x - y = (x - y)(x + y) + (x - y) = (x - y)(x + y + 1)
b) a2 - a - b2 - b = (a - b)(a + b) - (a + b) = (a + b)(a - b - 1)
c) - x2 - 6x - 9 = - (x + 3)2
d) ax + ay - 3x - 3y = a(x + y) - 3(x + y) = (x + y)(a - 3)
e) x2 - 4xy + 4y2 - 4 = (x - 2y)2 - 4 = (x - 2y - 2)(x - 2y + 2)