⇔x³ - 3x² + 3x² -9x + 21x -63+54≤0
⇔ x² (x-3)+3x(x-3) +21(x-3)+54≤0
⇔(x-3)(x²+3x+21+54)≤0
⇔\(\left[ \begin{array}{l}x-3 ≤0\\x²+3x+75≤0 \end{array} \right.\)
⇔\(\left[ \begin{array}{l}x≤3(tm)\\x²+3x+75>0(ktm)\end{array} \right.\)
vay btp co nghiem S={3}