a) $$3x - 3y = 3(x-y)$$
б) $$a^2 - 9b^2 = (a - 3b)(a + 3b)$$
в) $$a^2 + 4a + 4 = (a+2)^2$$
г) $$a(b-c) + 3(c-b) = a(b-c) - 3(b-c) = (b-c)(a-3)$$
д) $$3a + 3 - na - n = 3(a+1) - n(a+1) = (a+1)(3-n)$$