(1) (x+3y)^2-x-3y
=(x+3y)^2-(x+3y)
=(x+3y)(x+3y-1)
(2) ab(x-y)^2+b(y-x)^3
=ab(x-y)^2-b(x-y)^3
=b(x-y)^2[a-(x-y)]
=b(x-y)^2(a-x+y)