若变量x，y满足 则z=x-2y的最大值等于

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='32835' and id < 2617981 and id<>'2617981' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='32835' and id > 2617981 and id<>'2617981' order by id asc limit 5