"우리들이 대답해야 돼요."
Translation:We have to reply.
우리들이 - we
대답하다 - to answer; to reply; to respond
-야 되다/하다 - have to; need to; should (literally "to be becoming only if" since -야 approximates to "only if/when". Note that "to be becoming" is sort of like "to be ok")
-야 attaches to the -아/-어/-여 form of the verb (대답해 in this case)
All together you get "It is ok (돼요) only if (-야) we (우리들이) reply (대답해)", which essentially translates to and means "We have to reply."