Answer
$2x^2+5x+4$
Work Step by Step
$\bf{\text{Solution Outline:}}$
Combine like terms to simplify the given expression, $
3x^2-4x+4+9x-x^2
.$
$\bf{\text{Solution Details:}}$
By combining like terms, the expression above is equivalent to
\begin{array}{l}\require{cancel}
(3x^2-x^2)+(-4x+9x)+4
\\\\=
2x^2+5x+4
.\end{array}