When my washing machine drains it backs up into the kitchen sink. We have been on top of the house and snaked. There are no other drain problems in the house. Any suggestions?

Poor venting of that line

How do I fix that

I have a different take on your problem. It doesn't sound like a bad vent to me.
You have a blockage downstream from the kitchen sink.

We have been on top of the house and snaked. Go back and snake from the kitchen roof vent and this time put out enough snake to reach the clog. Put down enough snake to reach the base and 25 feet more. Good luck, Tom