若3x²-x=1求6x³+7x²-5x+1994的值

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6x³+7x²-5x+1994
=6x³-2x²-9x²-5x+1994
=2x(x²-x)-9x²-5x+1994
=2x-9x²-5x+1994
=-9x²-3x+1994
=-3(3x²-x)+1994
=-3x1+1994
=1994-3
=1991