6sin方a+sinacosa-2cos方a=0
(2sina-cosa)(3sina+2cosa)=0
2sina-cosa=0即tana=1/2或3sina+2cosa=0即tana=-2/3
因为a是2象限角所以tana=-2/3
sin（2a+3分之π）=sin2a*cos(π/3)+cos2a*sin(π/3)
=2tana/[1+(tana)^2]*1/2+[1-(tana)^2]/[1+(tana)^2]*√3/2
=2(-2/3)/(1+(-2/3)^2)*1/2+[1-(-2/3)^2]/[1+(-2/3)^2]*√3/2
=(5√3-12)/26