丨a3-2*a1,3*a2,a1丨
=丨a3,3*a2,a1丨-丨2*a1,3*a2,a1丨
=3*丨a3,a2,a1丨-2*丨a1,3*a2,a1丨
=3*(-1)*丨a1,a2,a3丨-0
=3*(-1)*丨A丨=6.