Question: how to get a customary answer from expansion

any of the numerical factors that multiply the successive terms in the expansion of an expression of the form  (x + a)^n,  for integral n,  in accordance with the binomial  theorem. These are any terms of the form
               ∑x^(n-k)*a^k*binomial(n,k)


how to get it.

convert((x+a)^n,Sum,x)


but ,i get a anwer that i don't want´╗┐

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