Answer
See below.
Work Step by Step
According to the Binomial Theorem we can obtain the $r+1$th term of the expansion of the binomial $(x+y)^n$ by the formula $_nC_rx^{n-r}y^r$
If the exponent on $(a+b)$ is $n$, then the exponents of $b$ start at $0$ and increase by $1$ on each term until they reach $k$ on the last term of the expansion.
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