Chapter 11 - Section 11.5 - The Binomial Theorem - Exercise Set - Page 664: 34

$-y^9$

Using the $(r+1)$st term of the expansion of $(a+b)^n$, which is given by $\dfrac{n!}{(n-r)!r!}a^{n-r}b^r$, then the $ 10 $th term of $ (5x-y)^9 $ is \begin{array}{l} \dfrac{9!}{0!9!}(5x)^{0}(-y)^{9} \\\\= -y^9 \end{array}