将x=6代入2/3(x-a)=a/6*x-1：
2/3(6-a)=a/6*6-1
2/3*6-2/3a=a-1
4-2/3a=a-1
5=5/3a
a=3
-2a^2+3a-4
= -2*3^2+3*3-4
= -18+9-4
= -13