|x+3|和(2x+y)²互为相反数,
即|x+3|+(2x+y)²=0
x+3=0, 2x+y=0
x=-3, y=-2x=6

3/4x²-(3y-1/4x²)+(y+1/2x)
=x²-2y+1/2x
=9-12-3/2
=-9/2