Will I have to attend regular classes if I have a back subject in engineering last semester in RGPV or could I just take the exams next time?

You don’t have to necessarily attend classes for back paper. You can easily give exams of backlog papers without attending classes.

With the new semester, your focus will be on it. It will be difficult to attend classes for both regular and backlog.

The best option is self-study. If it’s a numerical paper, practice a lot regularly. You may take help from faculty, your seniors or even your batchmate who is good in that particular subject. This will not be difficult in RGPV. A bonus tip is to solve previous year's papers. This will save you time and give you a better picture of your exam. 

Clear your back exams in time or you will face problems at the time of placement.