Answer
$-\dfrac{15}{y^3}$
Work Step by Step
Using the laws of exponents and the rule $a^{b}\cdot a^{c}=a^{b+c}$, the given expression, $ (5y^{-1})(-3y^{-2}) ,$ is equivalent to \begin{array}{l}\require{cancel}
(5)(-3)y^{-1+(-2)}
\\\\=
-15y^{-1-2}
\\\\=
-15y^{-3}
\\\\=\
-\dfrac{15}{y^3}
.\end{array}
For more complicated problems, it is helpful to remember that you can flip the location of numbers with negative exponents. In other words, you can move numbers with a negative exponent that are in the numerator to the denominator, and you can move numbers with a negative exponent in the denominator to the numerator.
