So you want to use this formula as a SSRS formula?

In that case, it would be something like this:

iif(Fields!event_type == "c","R" + Format(Fields!event_type,"####0"),iif(Fields!event_type == "i",Format(Fields!event_type,"####0"),"P" + Format(Fields!event_type,"####0"))

Not 100% sure about the format function though, so you better check if it works.

ps: ik ben van België, maar ik spreek Nederlands 🙂