(3x^2-xy^2)-3(x^2y-2xy^2) =3x^2-xy^2-3x^2y+6xy^2 =3x^2-3x^2y+5xy^2 =3x(x-y)+5xy^2 =3*(1/2)(1/2+1/3)+5*(1/2)(-1/3)^2 =1+5/18 =23/18
