show that
A=60, sin A=
sin C=
sin C=
sin C=
from here, work out cos C
cos C = (I am taking cos C to be positive,
otherwise angle B would come out reflex)
since A+B+C = 180, B=180-(A+C), sin B=sin (A+C)=sin A * cos C + cos A * sin C
=
Now use sine rule in the triangle ABC, you will get b=3c