3sina=sin(2b+a)
=>3sin[(a+b)-b]=sin[(a+b)+b]
=3sin(a+b)cosb-3sinbcos(a+b)=sin(a+b)cosb+sinbcos(a+b)
=>2sin(a+b)cosb=4sinbcos(a+b)
=>tan(a+b)=2tanb