I have to decide between root canal or extraction and implant, I have internal resorption of my molar.

If the tooth has internal resorption, a root canal may not solve the problem. It may continue to resorb any way. Get an opinion from an endodontist.  Some patients choose to try a root canal with the questionable prognosis. Eval what you would invest in a root canal and crown with buildup vs implant and crown abutment.  Then eval what you would loose if the endo treatment fails!!

I would have a consultation with an endodontist (root canal specialist) to find out what the long term prospects are for that tooth. It's usually best to keep your natural teeth as long as possible, but today's implants have a very high success rate. Implants also preserve bone and don't decay. It could be better and less expensive to have a successful implant rather than complicated dentistry on questionable teeth.

I agree with you that you should first confirm that you do have internal resorption. If you do have internal resorption, treatment success is very unpredictable. A dental implant, on the other hand, in the #30 location has about a 95% long-term success rate.