a) x2 - y2 - 2x + 2y = (x - y)(x + y) - 2(x - y) = (x - y)(x + y - 2)
b) 3a2 - 6ab + 3b2 - 12c2 = 3(a2 - 2ab + b2 - 4c2) = 3[(a - b)2 - (2c)2) = 3(a - b - 2c)(a - b + 2c)
c) x2 - 25 + y2 + 2xy = (x2 + 2xy + y2) - 25 = (x + y)2 - 52 = (x + y - 5)(x + y + 5)