s=3i−j;t=2i+3j
t−3s=(2i+3j)−3(3i−j)=2i+3j−9i+3j=−7i+6j
t+3s=(2i+3j)+3(3i−j)=2i+3j+9i−3j=11i
(t−3s).(t+3s)=(−7i+6j).(11i)=−77