\(x^3+y^3-3x-3y=\left(x+y\right)\left(x^2-xy+y^2\right)-3\left(x+y\right)\)
\(=\left(x+y\right)\left(x^2-xy+y^2-3\right)\)
\(x^3+y^3-3x-3y\)
<=> \( \left(x+y\right)\left(x^2-xy+y^2\right)-3\left(x+y\right)\)
<=>\(\left(x+y\right)\left(x^2+y^2-xy-3\right)\)