解
a²-3ab+9b²
=(-2)²-3×(-2)×(2/3)+9×(2/3)²
=4+4+4
=12

2(x-y)-3(y-x)
=2(x-y)+3(x-y)
=5(x-y)
=5×(-1)
=-5是对的吗对的