|A-3B|=|(-2A1,-2A2,-2A3,A4-3B4)|=(-2)³|(A1,A2,A3,A4-3B4)|=-8×[|(A1,A2,A3,A4)|+|(A1,A2,A3,-3B4)|=-8×2-3×|(A1,A2,A3,B4)|=-16-3×3=-16-9=-25