Answer
$2x^2+5x+4$
Work Step by Step
By combining like terms, the given expression, $
3x^2-4x+4+9x-x^2
,$ simplifies to
\begin{array}{l}\require{cancel}
(3x^2-x^2)+(-4x+9x)+4
\\\\=
2x^2+5x+4
.\end{array}
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