（3a-ab+7)-(-4a的平方+2ab+7)
=3a-ab+7+4a²-2ab-7
=3a-3ab+4a²
-3x-2(6x-5)
=-3x-12x+10
=-15x+10
2n-(2-n)+2(3n-1)
=2n-2+n+6n-2
=9n-4