Вот: <span>http://ru.static.z-dn.net/files/d85/e4c8c26c7ce845c0b01fada660a3cb51.doc</span>
=5y-(6y-(7y-8y+1))=5y-(6y-(-y+1))=5y-(6y+y-1)=5y-(7y-1)=5y-7y+1=-2y+1.