①原式=(-1/27)a^6*b^9+a^6*b^4.
②原式=(x-y)^3*[-(x-y)](x-y)^6=-(x-y)^10.
③原式=x^2-(4y-2z)^2=x^2-(16y^2-16yz+4z^2)=x^2-16y^2+16yz-4z^2.