先化简再求值：5x²-[3x-2(2x-3)-4x²],其中x=-1/2

5x²-[3x-2(2x-3)-4x²]
=5x²-[3x-4x+6-4x²]
=5x²-[-x+6-4x²]
=5x²+x-6+4x²
=5x²+4x²+x-6
=9x²+x-6
=9*(-1/2)²-1/2-6
=9/4-1/2-6
=9/4-2/4-24/4
=(9-2-24)/4
=-17/4