I have a greenhouse with the back (north) side against a garage. There is a vent between the garage & greenhouse. My question is, would you fan the hot air out of the greenhouse or draw cool air in from the garage? It gets too hot during the day even with door open. Thank you

If the garage is cooler I would pull from there. But once you exchange enough air it's not going to matter much. All the air will be at the ambient temperature. The key is to exchange as much air as possible.

If you aren't already you should consider using shade cloth on the top of your greenhouse.