Hi {{first_name}},
At [[event name]], you asked about [[specific question or topic they raised at your booth or session]]. I didn't have a complete answer in the moment, so I wanted to follow up properly.
[[2-3 sentence answer or explanation with a link to a relevant resource]].
If you want to dig deeper into how this would work for [[their company]], I'm happy to set up a call.
[[Your name]]