How to resolve System in QueryException: unexpected token: ',' message in Salesforce?

Try this.......... By using BackSlash  ''

public class ContactSearch {

public static list<contact> searchForContacts(string lastname ,string mailingCode)
{
String query = 'select id, name from contact where lastname ='' +lastname+ '' and MailingPostalCode = ''+mailingCode+''';
Contact[] myContacts = Database.query(query);
system.debug(myContacts);
return myContacts;

}

}