\[\boxed{\text{604\ (604).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[x = 5a^{2} + 6ab - b^{2};\ \ \ \ \]
\[y = - 4a^{2} + 2ab + 3b^{2};\ \ \ \ \]
\[z = 9a^{2} + 4ab\]