we have n(2n+1)(n+1) +n(2n+1) = n(2n+1)(n+1)+n(2n+1).1
= n(2n+1)(n+1 +1) = n(2n+1)(n+2)
P/s: sorry i don't know how to explain but i can give you some example so you can understand the question easily
3x(x+1) + (x+1).2 = (x+1)(3x+2)
(5x-4).1 - (5x-4).8 = (5x-4)(1-8) = 7(5x-4)
ok done. Thank to me :>