sin(2a-b)=3sinb
sin[a+(a-b)]=3sin[a-(a-b)]
sinacos(a-b)+cosasin(a-b)=3sinacos(a-b)-3cosasin(a-b)
4cosasin(a-b)=2sinacos(a-b)
[cosasin(a-b)]/[sinacos(a-b)]=1/2
tan(a-b)/tana=1/2