Ehm... don't?
For one: using the email subject could be tricky, because you can't guarantee it always has the number in there.
You could create a flow or script to find the number and the record and then reopen it. You would also need to open the request again and then what? New tasks? Tasks reopened?
How often does this happen? There is a reason for reopening. Either it wasn't done properly, which should have been checked with the user before closing, or the user forgot to add something, which would be a reason for them to open a new request.
Please mark any helpful or correct solutions as such. That helps others find their solutions.
Mark
