Сократите дробь
y-x-3y^2+3x^2/3x+3y-1

=(((3x^2-3y^2)-(x-y))/3x+3y-1=(3(x-y)(x+y)-(x-y))/3x+3y-1=
=((x-y)(3(x+y)-1))/(3(x+y)-1)=x-y