sin3acos6a+sin6acos3a=sin(3a+6a)=sin(9a)

sin(9*(3pi/8))=sin(27pi/8)=sin(11pi/8)

sin(11pi/8)=-sqrt[(1-cos11pi/4)/2]

cos(11pi/4)=cos(495)=cos(135)=-(sqrt2)/2

sin(11pi/8)=-sqrt[(1+(sqrt2)/2)/2]

Ответ:  -sqrt[(1+(sqrt2)/2)/2]