Chapter 11 - Section 11.5 - The Binomial Theorem - Exercise Set - Page 664: 32

$-20x^3y^3$

Using the $(r+1)$st term of the expansion of $(a+b)^n$, which is given by $\dfrac{n!}{r!(n-r)!}a^{n-r}b^r$, then the $ 4 $th term of $ (x-y)^6 $ is \begin{array}{l} \dfrac{6!}{3!3!}x^{3}(-y)^3 \\\\= -20x^3y^3 \end{array}