(a-1)-(3a²-2a+1)
=a-1-3a²+2a-1
=-3a²+3a-2
A=2(2-x)+1
=4-2x+1
=5-2x
代入A-2b=x-1
5-2x-2b=x-1
2b=-3x+6
所以b=-3x/2+3