\[\boxed{\text{518\ (518).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ (x + 5 - y)(x - 5 - y) = 0\]
\[\textbf{б)}\ \left( x^{2} + y^{2} - 4 \right)\left( y^{2} - 9 \right) = 0\]
\[\textbf{в)}\ (xy - 6)\left( x^{2} + y^{2} - 1 \right) = 0\ \]