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

(х-y)-(3х^2+3y^2)=(х-y)-3(х^2+y^2)=(x-y)-3(x-y)(x+y)=(x-y)-3(x+y)=3(x-y)(x+y)