(2x+1)²-(x-3)(2x-1)=3x
4x²+4x+1-(2x²-x-6x+3)=3x
4x²+4x+1-2x²+7x-3=3x
2x²+8x-2=0
x²+4x-1=0
x=-2±√5
解得x=-2-√5或x=-2+√5