∵ (x-3)²+(2y+1)²=0 两个非负数的和为0,这两个非负数都为0
∴x-3=0 且 2y+1=0
∴x=3,y=-1/2
∴x²-y³=3²-(-1/2)³
=9-(-1/8)
=9+1/8
=73/8