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 $_nC_rx^{n-r}y^r$.
Thus by the definition of the Binomial Theorem if the exponent on $(a+b)$ is $n$, then the exponents of $a$ start at $n$ and decrease by $1$ on each term until they reach $0$ on the last term of the expansion.
