Answer
$2x^6+9x^4+2x^3-4x^2+5x$
Work Step by Step
By combining like terms, the given expression, $
(3x^6+2x^4-x^3+5x)+(-x^6+3x^3-4x^2+7x^4)
$, simplifies to
\begin{array}{l}
(3x^6-x^6)+(2x^4+7x^4)+(-x^3+3x^3)-4x^2+5x
\\\\=
2x^6+9x^4+2x^3-4x^2+5x
.\end{array}