3[a-b]+[a+b]+[a-b/2]-[a+b/3]-[a-b/6]
=3a-3b+a+b+a-b/2-a-b/3-a+b/6
=(3a+a+a-a-a)+(b-3b-3b/6-2b/6+b/6)
=3a-8b/3