tan(a-π/3)=(tana-√3)/(1+√3*tana)=-√3/5
∴5tana-5√3=-√3-3*tana
∴tana=√3/2
∴sinacosa/（3cos^2a-2sin^a）=tana/(3-2tan²a)=(√3/2)/(3-3/2)=√3/3