2006-II-08

答案：D
$\begin{array}{rcl}
3x^2 & = & 21x \\
x^2 & = & 7x \\
x^2 – 7x & = & 0 \\
x(x-7) & = & 0
\end{array}$

$\therefore x=0$ 或 $x=7$。