Basement shower drain yet again. When water empties from washer, it comes up the shower drain, how do we get around that?

You piggybacked on a 5 year old thread so I gave you your own.
You have a blockage downstream from the shower.
This can be snaked from the shower drain, a stack clean out if there is one or from a roof vent. B If you snake from the roof put out enough snake to reach the base and 16 feet more. Good luck, Tom