Answer
$-5x^4-4xy-9y^3$
Work Step by Step
Removing the grouping symbols and then combining like terms, the given expression, $
(x^4-7xy-5y^3)-(6x^4-3xy+4y^3)
,$ simplifies to
\begin{array}{l}\require{cancel}
x^4-7xy-5y^3-6x^4+3xy-4y^3
\\\\=
(x^4-6x^4)+(-7xy+3xy)+(-5y^3-4y^3)
\\\\=
-5x^4-4xy-9y^3
.\end{array}
