大家帮我看看这些数学题对么？

12x-20x
=(12-20)x 对
=-8x

-5a+0.3a-2.7a
=(-5+0.3-2.7)a
=-2a 错-7.4a

x+7x-5x
=(7+1)x-5x
=8x-5x
=3x 对

1/3y-2/3y+2y
=(1/3-2/3+2)y
=-2又1/3y错5/3y

-6ab+ba+8ab
=(-6+8)ab+ba
=2ab+ba 错3ab

10y^2-0.5y^2
=(10-0.5)y^2
=9.5y^2 对

-5a+0.3a-2.7a
=(-5+0.3-2.7)a
=-2a
这题错了
-5a+0.3a-2.7a
=(-5+0.3-2.7)a
=-7.4a

-6ab+ba+8ab
=(-6+8)ab+ba
=2ab+ba
=3ab

其它题都对了。