You will need to setup a LINE login so that your users can connect to your app.
Then you can use the messaging API to setup a "bot" that will message the users once their question has been answered. Other option would be to setup things via LINE notify (https://notify-bot.line.me/), which seems to be a bot done for just that purpose.
Take a look on those two and see which better suits your specific needs.
Resources: https://developers.line.me/en/docs/line-login/overview/ https://developers.line.me/en/docs/messaging-api/overview/ https://notify-bot.line.me/en/