(3a+1)(2a-3)-6(a-1)(a+2)
=6a²-9a+2a-3-6*(a²+2a-a-2)
=6a²-7a-3-6*(a²+a-2)
=6a²-7a-3-6a²-6a+12
=-13a+9
当a=-3时,-13a+9=39+9=48