Ответ:
\[6xy(x - 1) - 3y(2x - 1) - y\left( 6x^{2} - y \right) =\]
\[= 6x^{2}y - 6xy - 6xy + 3y - 6x^{2}y + y^{2} =\]
\[= y^{2} - 12xy + 3y.\]
\[6xy(x - 1) - 3y(2x - 1) - y\left( 6x^{2} - y \right) =\]
\[= 6x^{2}y - 6xy - 6xy + 3y - 6x^{2}y + y^{2} =\]
\[= y^{2} - 12xy + 3y.\]