2006-II-08

Ans: D

\begin{array}{rcl} 3 x^{2} & = & 21 x \\ x^{2} & = & 7 x \\ x^{2} - 7 x & = & 0 \\ x (x - 7) & = & 0 \end{array}

$∴ x = 0$ 或 $x = 7$ 。