|A-B|
= |α-β,r2,2r3|
= |α,r2,2r3| + |-β,r2,2r3|
= (1/3)|α,2r2,3r3| + (-2)|β,r2,r3|
= (1/3)|A| - 2|B|
= 6 - 4
= 2