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