m-[n-2(m-n)]
=m-(n-2m+2n)
=m-(3n-2m)
=m-3n+2m
=3m-3n
=3(m-n)
=3[(-2ab+b^2-a^2)-(a^2+b^2-2ab)]
=3(-2ab+b^2-a^2-a^2-b^2+2ab)
=3(-2a^2)
=-6a^2
=-6*(-1/3)^2
=-2/3