Bonjour !
a1. BC = BA + AC (Chasles)
= -AB + AC (AB=-BA)
a2. 2BC + CA = 2(-AB+AC) + CA (question a1)
= -2AB + 2AC + CA
= -2AB + 2AC - AC
= -2AB + AC
a3. 2CB + 3BA + CA = -2BC - 3AB - AC
= -2(-AB+AC) - 3AB - AC
= 2AB - 2AC - 3AB - AC
= -AB - 3AC
b1. AB + AC = (AC + CB) + AC
= 2AC + CB
= -2CA - BC
b2. AC - 3BA + CB = AC - 3(BC+CA) + CB
= AC - 3BC - 3CA + CB
= -CA - 3BC - 3CA - BC
= -4CA - 4BC
b3. 2CB + 3BA + CA = 2CB + 3(BC+CA) + CA
= 2CB + 3BC + 3CA + CA
= 2CB + 3BC + 4CA
= -2BC + 3BC + 4CA
= 4CA + BC
N'hésite pas si tu as une question :)
