5a-{-3b+[6c-2a-(a-c)]}-[9a-(7b+c）]
=5a-{-3b+[6c-2a-a+c]}-[9a-7b-c]
=5a-{-3b+6c-2a-a+c}-9a+7b+c
=10b-3a-6c
注：别忘了写解,希望以后自己多动动脑筋