Answer
$7x^{2}-x+3$
Work Step by Step
The given expression, $
(21x^3-3x^2+9x)\div(3x)
,$ simplifies to
\begin{array}{l}
\dfrac{21x^3}{3x}-\dfrac{3x^2}{3x}+\dfrac{9x}{3x}
\\\\=
7x^{3-1}-1x^{2-1}+3x^{1-1}
\\\\=
7x^{2}-1x^{1}+3x^{0}
\\\\=
7x^{2}-1x^{1}+3(1)
\\\\=
7x^{2}-x+3
\end{array}
