5a+3[2b-3（a-b+2c）-（b+c）]
=5a+3(2b-3a+3b-6c-b-c)
=5a+3(-3a+4b-7c)
=5a-9a+12b-21c
=-4a+12b-21c
老大,化简就是这样了,不过你想干啥呢?