Code po300, car will not stay running

Is there any history to this? Has there been any recent repairs to the timing system? Any other repairs?

Here is the information for this code that we need to run down:

Also here is the guide we need to review for backfiring:

https://www.2carpros.com/articles/engine-backfires-while-running

The fact that it has this code and backfiring I would think there is more to the story so let us know about this info and we can go from there. Thanks