16-2-{-8-[-1-9-(3+6)]}=3还要先化简，在求值16+2a-{8a-[a-9-（3-6a）]}
=16+2a-[8a-(a-9-3+6a)]
=16+2a-(8a-7a+12)
=16+2a-a-12
=a+4
=3