要使-(3a-6)^2-1取最大值,则要使-(3a-6)^2=0
得a=2代入
4a-[-2a^2-(3a-2)]=4a-(-2a^2-3a+2)
=4a+2a^2+3a-2
=7a+2a^2-2
=14+8-2=20