Answer
$x^4+3x^3$
Work Step by Step
By combining like terms, the given expression, $
3x^3-x^2+x^4+x^2
,$ simplifies to:
\begin{array}{l}\require{cancel}
x^4+3x^3+(-x^2+x^2)
\\\\=
x^4+3x^3+0x^2
\\\\=
x^4+3x^3
.\end{array}
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