lim x→0 1/x( a/x-b/sinx )
=lim x→0 xsinx/(axsinx-bx^2)
=lim x→0 (xsinx)'/(axsinx-bx^2)'
=lim x→0 (sinx+xcosx)/(asinx+axcos-2bx)
=lim x→0 (sinx+xcosx)'/(asinx+axcos-2bx)'
=lim x→0 (cosx+cosx-xsinx)/(acosx+acos-axsinx-2b)
=(1+1-0)/(a+a-0-2b)
=1/(a-b)=-1/6
a-b=-6