The answer to 'how to clean sewer line' is to use a sewer snake or plumber's auger to clear any blockages in the sewer line. This involves feeding the snake or auger into the sewer cleanout or drain and working it through the line to break up and remove any clogs or debris.
It's important to wear protective gear like gloves and eye protection, and to work slowly and carefully to avoid damaging the pipes. You may also need to use a sewer camera to inspect the line and identify the location of the blockage. If the clog is severe or the line is damaged, it's best to call a professional plumber for assistance.