Please you apologize me by my English.
I don't know how make that: $$(n+a)^b = \Theta(n^b), b > 0$$ I know, I must to find two constants such that: $$ c_{1} n^b \leq (n+a)^b \leq c_{2} n^b $$
I do not know what else to do. I'v tried with the Newton's binomial, but I'm lost.