\[x^{3}y^{3} - x^{2}y^{3} + xy^{6} + \ *\]
\[*\ = - xy^{6}:\]
\[x^{3}y^{3} - x^{2}y^{3} + xy^{6} - xy^{6} =\]
\[= x^{3}y^{3} - x^{2}y^{3}.\]