Subwoofer connection issues

Hi guys
Pls help out
I want to connect subwoofer to cubase 9.5 through Fireface 800
I think I need to do something with audio connections menu
I have connected it to Output 3 and 4 in fireface. What do I need to do in cubase to direct sound. I made 2 hubs in outputs, but no sound coming out
Thanks

Alex