they lost 2-0 in away leg which means they can advance by the following
1) win by 3 or more goals
2) win 2-0 then go to extra time or penalties and then must win that as well
if Zenit scores 1 goal then Liverpool must score at least 4 to advance. a 3-1 Liverpool win means teams are 3-3 on aggregate but Zenit advances due to more away goals
Clubs play two matches against each other on a home-and-away basis. The
club which scores the greater aggregate of goals qualifies for the next
round, with away goals and then penalties used to determine the winner
in the event of a draw.