Even my teacher could not do this.This is a question extracted from the mathematics challenge board created by my friend in school.This is the question
Given $x-y=3$,find the value of $x^3-2x^2y+xy^2-3xy+3y^2$
The way I do this first,is that the first 3 terms I factorise,the last 2 terms I also factorise by the highest common factor
\begin{align} x^3-2x^2y+xy^2-3xy+3y^2 & = x(x^2-2xy+y^2)-3y(x-y)\\ & = x(x-y)^2-3y(x-y)\\ \end{align}
The problem is just,how do I get $x-3y$?