Answer
$-4x^3y-x^2y^2+xy^3-y^3+19$
Work Step by Step
Combining like terms, the given expression, $
2x^3y-y^3+xy^3+8-6x^3y-x^2y^2+11
,$ evaluates to
\begin{array}{l}\require{cancel}
(2x^3y-6x^3y)-x^2y^2+xy^3-y^3+(8+11)
\\\\=
-4x^3y-x^2y^2+xy^3-y^3+19
.\end{array}
