Answer
$-\dfrac{63}{y^7}$
Work Step by Step
Using the laws of exponents, the given expression, $
(-7y^{-3})(9y^{-4})
,$ is equivalent to
\begin{array}{l}\require{cancel}
(-7)(9)y^{-3+(-4)}
\\\\=
-63y^{-3-4}
\\\\=
-63y^{-7}
\\\\=
-\dfrac{63}{y^7}
.\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.
