if i am taking back up of content databases of all web application and configuration database from SQL server(i am not using any other back up method like back-spfarm command etc.), then can i restore my entire farm?
You'll have to first build a new SharePoint farm to attach the restored content databases to in order to restore the content of your web applications, you won't be able to use the back up of the configuration database for anything other than a point of reference (and even that will be difficult b/c you'll have to do it via SQL queries and trying to figure out the structure of a SharePoint database can be like learning Sanskrit: difficult, provides little value, and highly unrecommended).
Microsoft does not support the practice of restoring a SQL Server backup of a configuration database into a farm. You'll have to be able to rebuild a new SharePoint farm with all the same settings, configurations, and deployed customizations as the original farm, create new web applications, then attach your restored content databases to those new web applications.
Because of this, SharePoint backup and restore (out of the box) is about protecting your content first and foremost, and then knowing how to build a new farm to restore that content into.