3x²-｛5x²-【2x-（3x²-3x+x²）】+5x｝
=3x²-5x²+【2x-（3x²-3x+x²）】-5x
=3x²-5x²+2x-（3x²-3x+x²）-5x
=3x²-5x²+2x-3x²+3x+x²--5x
=-6x²
x=二分之一时
原式＝－3/2