How to use a multiple linear regression model to investigate whether the effects of A on B are different for X defined in the variable X?

If you tell us where you get stuck, its possible we might help.

I would really encourage you to review the following guide, FAQ: Tips for writing R-related questions.
For example, the guide emphasizes asking coding questions with formatted code-chunks and a reprex.

You may have noticed folks here requesting minimal reprexes, that's because asking questions this way saves answerers a lot of time.

Reproducible Examples:

  • help make your question clear and replicable
  • increases the probability folks will reach out and try to help,
  • reduces the number of back-and-forths required to understand the question,
  • and makes your question and suggested solutions more useful to folks in the future researching similar problems.