$\begin{array}{l} {\left( {x - y} \right)^3} + 6{\left( {x - y} \right)^2} + 12\left( {x - y} \right) + 8\\ = {\left( {x - y} \right)^3} + 3.2.{\left( {x - y} \right)^2} + 3.\left( {x - y} \right){.2^2} + {2^3}\\ = {\left( {x - y + 2} \right)^3} \end{array}$