\[\boxed{\text{593\ (593).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ M + \left( 5x^{2} - 2xy \right) =\]
\[= 6x^{2} + 9xy - y^{2}\]
\[= x^{2} - y^{2} + 11xy\]
\[M = x^{2} + 11xy - y^{2}.\]
\[\textbf{б)}\ M - \left( 4ab - 3b^{2} \right) =\]
\[= a^{2} - 7ab + 8b^{2}\]
\[= a^{2} - 3ab + 5b^{2}\]
\[M = a^{2} - 3ab + 5b^{2}.\]
\[\textbf{в)}\ \left( 4c^{4} - 7c^{2} + 6 \right) - M = 0\ \]
\[M = 4c^{4} - 7c^{2} + 6\]