Guest room bath tub backs up

Our guest room bath tub will gurgle and sometimes back up with what looks like lint etc, when our washing machine (on the same floor), drains.

The shower is hardly used.

I had read another thread on here that the shower should be run maybe once a week? We did however have a guest over last week that used the shower for 4 days. It did the gurgling/back up earlier today.

Any suggestions would be greatly appreciated.

The drain is clogged. Call a plumber to auger it. Do not use chemicals. The suggestion to run the shower every few days is to keep the p trap full of water to block sewer gas and has nothing to do with a backed up drain.

Thank you for the tip.