6X-28-2.5X+30.5=（6-2.5）X+（30.5-28）
=3.5X+2.5
8A-(5A+8)=(8-5)A+8
=3A+8
（5x+7)-(4X+0.18)=5X-4X+7-0.18
=X+6.82
6A+12A=18A