You need to have a plumber come out and use a roto-rooter the sewer line. I was quoted $150 for that job. Once the block is bad enough, I'm not sure there's any way to remove the roots chemically. If you're not removing the tree, it will likely be a recurring problem. Depending on the material used to construct the sewer line (PVC, crock, etc.), it may be beneficial to just replace the line. For instance, the home I recently purchased has a sewer line made of crock, which is more susceptible to infiltration by roots. In fact, I already know it has roots in the sewer line, as I had a sewer camera inspection before purchasing. I am going to just suck it up and replace the line with PVC, which should be a more durable option. I know that's not the answer you wanted to hear, but I hope it helps. Good luck!