解(2x+1)(2x-1)-(x+1)^2+2x
=4x²-1-(x²+2x+1)+2x
=3x²-2
由x^2-12=0
即x^2=12
即(2x+1)(2x-1)-(x+1)^2+2x=3x²-2=3×12-2=34.