Chapter 5 - Section 5.7 - Negative Exponents and Scientific Notation - Exercise Set - Page 408: 40

$-\dfrac{5}{x^5}$

Using the laws of exponents, the given expression, $ \dfrac{-15x^4}{3x^9} ,$ simplifies to \begin{array}{l}\require{cancel} (-15\div3)x^{4-9} \\\\= -5x^{-5} \\\\= \dfrac{-5}{x^5} \\\\= -\dfrac{5}{x^5} .\end{array}