6sin^2a+sinacosa-2cos^2a=0
(2sina-cosa)(3sina+2cosa)=0
∵a∈[π/2,π)
∴2sina-cosa>0
3sina+2cosa=0
tana=-2/3
cot(-a-π)sin(2π+a)/cos(-a)tana
=-cotasina/cosatana
=-tana
=2/3