2／3a-{1／3－[(3a²-5a+1）－1／3a－5]}
=2／3a-1／3+[(3a²-5a+1）+1／3a+5]
=2／3a-1／3+(3a²-5a+1）+1／3a+5
=2／3a-1／3+3a²-5a+1+1／3a+5
=3a²-4a+17/3