6x-3x²-7
=-3x²+6x-7
=-3(x²-2x) -7
=-3[(x²-2x+1) -1]-7
=-3(x-1)²+3-7
=-3(x-1)²-4≤-4＜0
∴无论x取什么实数,6x-3x²-7的值都小于0.