|a1+a2,2a1,a2-a3|
=|a1,2a1,a2-a3|+|a2,2a1,a2-a3|
=0+|a2,2a1,a2|-|a2,2a1,a3|
=0+0-2|a2,a1,a3|
=-2|a2,a1,a3|
=2|a1,a2,a3|
=6